THE STEM OF A PLANT

The stem of a plant, unlike the root, bears lateral appendages that are different in form from the axis that produces them – that is, the stem bears leaves.

In the leaf-axil, the angle which the leaf makes with the stem, there is commonly a leaf-bud. This is a branch in miniature, a branch whose axis is extremely short and whose leaves, therefore, are very closely packed. The youngest leaves bend over the tip of the rudimentary stem and so protect the growing point.

A Brussels Sprout, which grows in the axil of a leaf of one variety of Cabbage, is so big that bud structure is very plainly seen in its longitudinal section.

The basal leaves of the bud are the largest. At the tip tiny crinkled leaves curve over the growing point. In the axils of most of the leaves there are well-formed axillary buds. A careful dissection of one of the largest of these shows the same structural plan as that of the mother-bud.

The buds of trees have a long spell of waiting to pass through. They are formed early in the summer and do not expand until March or April of the following year.

To fit them for this resting stage they are not naked, as is the Brussels Sprout, but are enclosed in a certain number of scale-leaves.

On the outside of the bud of the Horse Chestnut there are dark, tough scale-leaves whose stickiness gives them most effective additional protection against external ills – against cold and wet, and against attacks of fungi and micro-organisms generally. At the same time the scale-leaves make it impossible for the rudimentary foliage leaves they enclose to lose water.

In dissecting this bud it is necessary, now and again, to dip both it and the dissecting needle into methylated spirit. The stickiness dissolves in the alcohol, and so the work of dissection is made easy.

For the purposes of dissection it is well to choose a terminal bud, that is, one that is not developed in a leaf-axil, but is a direct continuation of the branch. The scale-leaves grow in pairs on the short axis of the bud and are disposed at right angles to one another. The scales at the base of the bud are very small. The succeeding scales are increasingly larger. The last pairs are not only very large, but are completely circular, or even overlapping, so that their removal is a little difficult. When they are removed, the methylated spirit is finished with – its further use only makes the remaining leaves of the bud messy and bedraggled.

These remaining leaves are two pairs of foliage leaves, each leaf being possibly about the size of the little finger-nail. The fluffy hairs that cover them afford still further protection. When this fluff is removed the form of the foliage leaf is revealed. It is made up of slender leaflets, five, seven, or even nine in number. Each leaflet is folded book-wise, along the middle line.

The tip of the axis of this particular bud bears a roundish mass of flower-buds that expand in the springtime into the beautiful Horse Chestnut inflorescence that stands erect, like a well-fixed candle on a Christmas tree.

If Horse Chestnut twigs are kept in water in the laboratory the buds will burst, in the warm room, some time before they expand in nature. Every detail of their opening may then be seen. The stickiness melts and the scale-leaves are pushed apart. Later they bend slowly backwards until their final position is directly opposed to their original one and the tips of the leaves point down.

In the meantime the axis has not only elongated, but it is much thicker and there has been a rapid elongation of the leaf-stalks. When these form an angle of 90 degrees, or somewhat less, with the axis, the blade of the leaf is like a baby’s hand inside the fingerless baby’s glove. The blades then lose the protecting hairs and turn green in the light. The leaflets spread out in the finger-like manner that has given the name palmate to the compound leaf of the Horse Chestnut.

While the foliage leaves have been expanding in this way, the scale-leaves, one by one, have dropped off. They were so numerous, so strong, and, in some cases, so spreading at the base, that when they fall they leave ring scars, or girdle scars, at the base of the bud axis that supported them.

Further development brings the leaves and flowers to their perfection towards the end of May. The last Sunday of this month is Chestnut Sunday. Throughout the summer the leaves, stems, and flowers carry out their various functions, and by September the conkers, the seeds in the fruits of the Horse Chestnut, are formed. Although there were so many flowers on the long axis of the inflorescence, only two or three fruits are formed on each spike, because many of the flowers are male only, that is, they have stamens but no carpels.

Obviously the development of flowers at the tip of the bud makes any further growth in length at this point quite impossible. If the matter were left here it would mean that our Horse Chestnut trees, once they were old enough to produce flowers, would have many very curiously stunted branches. Compensation is, however, found in a most interesting way.

Terminal buds of the Horse Chestnut that contain a rudimentary inflorescence have, except in rare cases, only two pairs of foliage leaves. When such a bud bursts and its axis elongates, axillary buds develop very quickly in the axils of the upper pair of foliage leaves. These are naked buds. As they are not going to persist through the winter, they do not need protecting scale-leaves. It would, in fact, be bad economy for them to be so hampered, because it is their business to grow into leafy stems in the shortest possible time. As no fall of scale-leaves precedes their expansion, naturally there are no girdle scars at the bases of these stems which may grow to a length of 2 or 3 inches, bearing two or three pairs of small foliage leaves and ending in a terminal bud. The growth of these short, lateral branches is seen most plainly in autumn when the foliage leaves have fallen. If both buds have developed, the stems project, one on either side of the saddle scar left by the inflorescence. Excellent catapults are made of such forked stems.

In the Horse Chestnut it is not usual for the axillary buds to contain flowers. Nor, of course, are they found in all terminal buds. In such flowerless buds the number of foliage leaves is greater and the growing point retains its individuality. It is free to go on growing. If it, in its turn, terminates in a flowerless bud another length of stem is added the following year, still in the same straight line – and so on. The production of such a definite one limb is known as monopodial development.

When monopodial growth is interrupted by any cause, as by flower production in the Horse Chestnut, the axis that continues the onward growth is a side branch of the main axis, and at first there is an angular interruption in the straightness of the branch. Later growth probably smooths out the angularity, so that a whole long branch, which may be made up of a branch of a branch, appears monopodial, whereas it is by no means a continuous axis Such a branch is termed a sympodium.

The growth then, in the Horse Chestnut, is both monopodial and sympodial. The great majority of the buds in this tree are permanently dormant.

The branching of the Sycamore closely resembles that of the Horse Chestnut. In the Ash the black terminal bud always continues the onward growth of the axis, which is, therefore, a monopodium.

The difference between the Brussels Sprout bud and that of the Horse Chestnut is only one of degree. Like some of the Horse Chestnut buds the Brussels Sprout is naked. As a matter of fact the bud scales of the Horse Chestnut are themselves the bases of undeveloped foliage leaves. That this is so is seen by the fact that the inner scale-leaves often divide into tiny leaf-blades at their tips.

At first sight a more fundamental difference seems to be the presence of axillary buds in the Brussels Sprout. This, however, is but an illusory distinction. When a longitudinal section of a Horse Chestnut bud is examined with the microscope, buds are distinguished in the axils of the foliage-leaves. They are at this stage extremely rudimentary, but they are definitely present.

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