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Fruit Key and Twig Key to Trees and Shrubs
By William M. Harlow
Dover Publications, Inc.Copyright © 1946 William M. Harlow
All rights reserved.
The narrow-leaved evergreens, or conifers, belong to a group of trees, ancient in lineage, known as the Gymnosperms. This name was happily chosen since it means "naked seed", and is thus descriptive of what we find when we examine the fruit. All of the conifers, included in the key, produce cones, even though in one or two cases, the appearance is more like that of a "berry". When the cone scales open, the seeds are revealed lying "naked" upon them, and not surrounded by any sort of containing structure.
The life history of cones, especially those of the pines, has some interesting chapters. Like most if not all of the conifers, the brilliant red female flowers are borne erect on the branch. By the end of the first growing season they become small inconspicuous green cones. Either that autumn or the following spring, these slowly turn over, assume a pendent position, and during this second season grow to maturity. They may then either open and release their seeds, or remain closed for months or years, depending upon the species. Pitch pine cones open gradually during the winter, and drop their seeds upon the snow where many are eaten by birds who would otherwise find it more difficult to obtain food. The cones of the jack pine often remain closed for many years, and require heat such as that from a moderate ground fire, to open them. When this happens, the stored seeds of perhaps twenty years are released nearly at the same time and this may result in dense even-aged stands of young trees. In the absence of sufficient heat, some of the closed cones become entombed in the trunk as the growing tissues cover them over completely. Later, when the trunk is sawed, these embedded cones are exposed, and some of their seeds will germinate when planted.
Not all of the "conifers" have cones. The yew (shrubby in the region covered by the key) has a bright scarlet fleshy seed. The flesh is edible, with a pleasant but somewhat sickish-sweet taste. The seed itself, however, should not be cracked between the teeth since the "meat" within, like the leaves and other parts of the plant, contains a bitter poisonous alkaloid.
The broadleaved trees belong to another group of plants known as the Angiosperms in which the seeds are borne enclosed in an ovary. The Angiosperms are divided into the Monocotyledons, such as the grasses, corn, and palms; and the Dicotyledons including thousands of herbs, vines, shrubs, and trees. Compared to the more primitive conifers, the Angiosperms present an amazing array of diverse fruit forms, and it is in this group that a proper definition of the word "fruit" becomes more difficult.
The characteristic female flower structure of the broad-leaved tree is the pistil. Sometimes the pistil is easily seen and recognized with the naked eye. It may, however, be extremely small so that a hand lens is necessary. Further, many pistils may occur in a compact cluster so that one is not readily distinguished from another. To name properly the type of fruit, it is often necessary to trace its development from the pistil.
The lower part of the pistil is called the ovary. It contains a cavity, and attached to the walls of the cavity are the ovules which upon fertilization become the seeds. The cavity may be single, or it may be divided into several smaller chambers by partitions. When so divided, the ovary or pistil is called compound. The simple type has no partitions, but in certain species a single cavity may not signify a simple pistil. A definition of the word fruit may now be proposed. A fruit is a ripened ovary together with any appendages or dried flower parts which it still may retain. Notice that the flower develops into the fruit, the fertilized ovule becomes the seed. This is important to remember, and when you look at something or other of a fruiting nature you must always ask yourself the question "Is this a fruit or is it a seed, and how do I know which it is?" Unless you are this critical you will soon be lost in the maze of multitudinous fruit and seed forms of even our common trees.
When several to many pistils are compacted, the result is a compound fruit. These are of two sorts. In such trees as magnolia, and yellowpoplar (Pages 27 & 35) many pistils are borne in a cone-like structure within asingle large flower. The resulting fruit is said to be aggregate. The other condition is where a number of small separate flowers each with its pistil (s) are borne close together. Here the result is a multiple which characterizes such trees as the sycamore and sweetgum (Page 35).
Fruits which develop from a single pistil may be classified in one of several ways. They may be dry, or fleshy, but in certain cases this is not easy to ascertain, since the flesh not only may be thin, but also so hard and firm that it is not recognized as flesh. If fleshy, the fruit is probably adrupe, berry, or pome. It is easier to give well known examples of these than it is to give definite features by which an unknown fruit may be classified. Again this is because it is necessary to trace the development of the fruit from its precursor, the flower.
Drupe – Botanically, this is a fleshy fruit in which the inner ovary wall is hard and bony, the outer one soft and fleshy. Common examples are the prune, cherry, peach, plum, and almond. A drupe usually has one pit, but may have two. Some fruits such as those of the walnuts and basswoods are said to be "drupaceous". This means that man confronted with a tremendous number of natural forms, tries to pigeonhole them for easy reference, but every once in a while he comes across some form which is neither one thing nor the other. In the cases mentioned, the "flesh" is almost dry, or nearly hard and woody, hence the term drupaceous which is merely a device for getting out of an awkward situation.
Berry – The definition is easy, but the term is greatly corrupted due to popular usage. A berry is a fleshy fruit in which both inner and outer ovary walls are fleshy, and the seeds are distributed throughout. Some common examples are blueberry, and huckleberry (these are not synonymous), gooseberry, tomato, and persimmon. Strawberry, blackberry, raspberry and mulberry are not "berries", according to the above definition, but are compound fruits made up of many small units (mostly druplets). Careful inspection will show that this is so. Although a berry is usually several-seeded, it may contain only one seed. If this seed has a hard heavy seed coat, the fruit may be mistaken for a drupe.
Pome – Here, the outer ovary wall is fleshy, the inner one papery, or like cartilage. Everyone, in eating apple pie, has rasped his tongue with these sharp cartilaginous portions of the inner ovary wall incompletely removed by the cook, or apple processer. Incidentally the pome is derived from a compound pistil, and the enlarged receptacle is also a part of it (slice an apple in half lengthwise and see for yourself). The drupe and berry are from simple pistils. Some common pomes in addition to the apple are those of pear, shadbush, mountainash, and hawthorn (thornapple).
Dry fruits are conveniently classified by observing whether at maturity they split along definite seams (sutures). If they split, they are called dehiscent, if they do not, they are said to be indehiscent. The three common dry dehiscent fruits are the legume, follicle, and capsule.
Legume — This is the product of a simple pistil, and cracks open along two sutures. Examples include the bean, pea, clover, blacklocust and coffeetree.
Follicle – This fruit, also from a simple pistil breaks along only one suture. Single follicles are not found on the trees covered by this key, but in magnolia the fruit is an aggregate of follicles. (Page 35)
Capsule – The capsule, in contrast to the legume and follicle, comes from a compound pistil, and may open in one of several ways. Common examples are the poppy, lilac, catalpa, and horsechestnut, even though most people think of the latter as a "nut".
The dry indehiscent fruits found on trees covered by the key, are the achene, samara, and nut.
Achene — This is a small, unwinged, but sometimes plumed, one-celled, one-seeded fruit. In sycamore, many achenes are compounded to form the globose head characteristic of this tree. (Page 35)
Samara — The samara is similar to the achene except that it has a thin membranous wing extending terminally, or surrounding the seed cavity. Common examples are those of the maple, ash, and elm.
Nut — Botanically, the nut is not difficult to define, but it is not always easy to recognize, or to separate from certain other types. A typical nut is partially or wholly enclosed in a husk which may be papery, leafy, woody, or spiny in character. The nut itself has a bony, or leathery outer wall and is usually one-seeded by abortion. This means that more than one ovule may be present but commonly only one develops. Examples include the chestnut, oak (acorn), beech, hickory, and hornbeam. Superficially, the fruit of the horsechestnut would seem to fit the description, but actually there is no husk, and that which appears to be a husk is the outer ovary wall of a capsule. This is a very good example of the fact that things are not always what they seem to be, and highlights the importance of not making snap decisions before getting all available evidence.
Although autumn would seem an appropriate time to look for fruits and seeds, certain species fruit in the late spring. Included in this group are the three northeastern elms, the soft maples (red and silver), red birch, and the willows and poplars. Most of these trees are commonly found along water courses where the spring-maturing seeds find fresh, moist, alluvial soil waiting to receive them. Seeds of the willows and poplars germinate within two or three days, an interesting fact which can be verified by placing a few of them upon a moist blotter covered with a water glass.
Trees usually do not produce seed until they have reached a considerable size, but in a few species such as jack pine, cones may be produced when the tree is only two to three feet high.
There may be more than one answer to the question "why doesn't my tree bear fruit?" These are some of the possibilities:
(1) the tree still is too young; (2) male flowers are borne on trees separate from those bearing female ones and the tree in question may be a male; (3) a late frost destroyed the flowers; (4) it may be an "off" year. Some species produce flowers and fruit every year, but others are characterized by extremely heavy crops followed by several seasons when not a single flower can be found on an entire tree. Sugar maple is one of these, and over large areas almost every tree will appear covered with a bright yellow "mist" caused by countless thousands of flowers. Two or more years will then pass before the next appearance of any flowers or fruit at all. Usually an occasional tree is "out of step" with the rest, and such an individual flowers and fruits during the "off" years.
HOW TREES TRAVEL.
You may say that trees do not travel, since one of the most obvious differences between the higher plants and animals is that the plants remain rooted while the animals are free to move from one place to another. Although an individual tree, in the absence of accidents (including man), may grow to maturity on the spot where the fruit or seed was deposited, one or both of these structures often have developed special ways of travelling long distances. By means of their offspring, then, trees are able to move about and change their location from one generation to the next.
For many millions of years the fruits and/or seeds of certain species have been airborne, often for considerable distances. Some, like those of the ashes and maples, have thin, membranous wings, and although these fruits are relatively heavy, they may, if released at a sufficient height, ride a strong wind a hundred yards or more.
The flask-shaped fruits of the willows and poplars release tiny seeds which are about the size of a pinhead. These are equipped with a parachute of silky or cottony floss, and will float for many miles, even in a light breeze. Some of the tropical jungle trees have remarkable airborne seeds. One of the writer's students found himself surrounded by graceful, silvery-winged "gliders" with a five inch wing span, sailing and dipping downward from the tops of tall trees in the New Guinea forest.
Some trees rely upon another sort of wing for the dispersal of their fruits or seeds. It is no accident that common hedgerow species include the various cherries, dogwoods, poison-ivy, buckthorn and wild grape. Sometimes certain shrubs are found in seemingly strange places. When scouting for gooseberry or currant bushes (white pine blister rust control), it is customary to scrutinize the lower forks of old trees since Ribes bushes are commonly found there. The writer knows of one gooseberry bush which has flourished for at least twenty-five years in the fork of an old sugar maple. Nearby are similar "sites" bearing red elder bushes ten feet above the ground. The only reasonable explanation seems to be that the seeds were left there by birds.
Sometimes flocks of starlings roost by the thousands in young pine plantations. Just at dusk these birds blacken the sky and disappear into the tops of the trees until it is hard to see how any more could be sheltered. After dark if one approaches the roosting birds, and raps one of the trees with a stick, there is a roar of wings which must be heard to be appreciated. The area smells like a chicken roost, and considerable nitrogenous material is deposited, so much so that whole sections of the plantation may be killed. A year or more later, thousands of seedlings spring up, principally those of buckthorn, but the cherries, mulberry, and other fleshy fruited species are also found. Within a few years there results a thicket of "weed" trees through which it is difficult to walk. The fact that birds eat certain fruits has thus resulted in a complete change from a pure coniferous stand to one of hardwoods (broadleaved), and worthless ones at that.
Fruit or seeds without wings are distributed in other ways. A great many acorns and other nuts are planted by squirrels even though they exact a considerable levy in so doing. Acorns of the white oaks germinate in the autumn and almost depend upon the squirrels to plant them before they are frozen. Large fruits like those of the walnuts and hickories may roll downhill, and in this way "travel". Under proper conditions, many fruits may float or be carried by running water and eventually germinate many miles away from the parent tree. Finally, perhaps the greatest seed dispersing agent is man himself who crosses oceans and continents, and mixes introduced species with the native until an observer lacking adequate records might not be able to tell one from the other.
POISON-SUMAC Toxicodendron vernix (L.) Kuntze
Although the ripe whitish fruit is said to be devoid of the poisonous resin found in special canals scattered throughout the rest of the plant, it is thought best not to include the fruit in the key. Careful scrutiny of the illustration on Page 14 will enable anyone to recognize these fruits. They are borne on a large swamp shrub or small tree, with alternate compound leaves which turn to a brilliant scarlet in autumn. If any part of the plant is bruised and the canals broken so that the sticky resin touches the skin, dermatitis will probably result.
HOW TO USE THE KEY
When the user must choose one of two alternatives, a key is said to be dichotomous, and this is the type used here. Let us suppose that you have picked up the fruit illustrated below, and wish to discover its identity. Turn to the next page and read the two alternatives offered at No. 1. If you have studied the "Introduction" you will see at once that the fruit is not a cone, and you are directed to turn to No. 27. The fruit is not winged, hence you follow the second part of No. 27 to No. 44. At No. 44, it is evident that your unknown fruit is not fleshy, and this brings you to No. 76. The fruit is not compound and so you next consult No. 80. At No. 80 it is evident that the fruit is not an acorn, and the second part of No. 80 brings you to No. 97. Here it is clear that the fruit is not a pod, and it is hoped you will recognize it as a leathery nut enclosed by a husk. If so, you should proceed to No. 98. The fruit is more than one half inch in diameter which takes you to No. 101. Since the husk splits, 102 is consulted. The nut is encased in a prickly husk, so you turn to No. 103. This must be the end of the trail since there is no way of going further. The nut is triangular in cross-section, and after comparing the unknown with the illustrations, there should be no doubt concerning its identity.
Excerpted from Fruit Key and Twig Key to Trees and Shrubs by William M. Harlow. Copyright © 1946 William M. Harlow. Excerpted by permission of Dover Publications, Inc..
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