Field and Woodland Plants

by William S. Furneaux

We have spoken of the protection afforded to the dormant bud during the winter period, but it is interesting to note that protection is necessary for the young leaves even after they have forced themselves well out into the light and air. The reason for this is that the epidermis or outer skin of the young leaf is not properly developed. It is not yet water-tight, and, consequently, the sap of the tender leaves would rapidly evaporate, so that they would soon become dry and shrivelled.

The means by which the young leaves are protected will be readily seen if we watch the gradual development of the bud. In many cases these leaves remain folded long after they have left the shelter of the original bud-scales, the manner of folding being the same as that which obtained while within the bud. Sometimes they are folded like a fan, or like the leaves of a book; sometimes rolled one within the other, or irregularly crumpled in such a manner that nothing is exposed to the air except the edges of the leaves and the surfaces of the veins.

In addition to the protection from evaporation afforded by the folding of the young leaves, many are covered with a dense coat of "wool." Young leaves of the Horsechestnut are very thickly covered with such a coat, of which only the slightest traces are to be seen in the fully-grown leaf. The young leaves of the Beech are folded like a fan for some time after they have left the enclosure of the bud, and the folding is such that the only parts exposed are the margins, the midrib, and the strongly-marked parallel veins. But since all these parts are provided with hairs, the young leaf, as long as it is folded, is surrounded by a complete protective covering. As the epidermis develops, and the danger of loss by evaporation thus reduced, the leaf straightens itself out, and the hairs either fall or become shrivelled. The leaf of the Wayfaring Tree is protected, while young, by a complete covering of starlike hairs which form a fine felted coat over the whole surface; and when the epidermis is properly formed, the hairs are all shed.

Some young leaves are preserved by scaly stipules which surround them after they have emerged from the bud; and as soon as the epidermis is sufficiently impermeable the stipules, having done their work, fall to the ground. So great is the shower of these transient structures, in the case of the Oak, Elm, and Lime trees, that the ground is almost completely covered by them.

Twig of the Lime in Spring, Showing the Deciduous, Scaly Stipules.

Young leaves have yet another way of preventing the evaporation of their sap, and that is by turning themselves into the erect position so that the warmth of the spring sun has but little effect on them. The young leaves of various grasses turn their apices upwards; while those of the Horsechestnut, after having lost the protection afforded by the woolly covering and the original folding, turn themselves with their points downwards. Later, when the epidermis is well formed, and the leaves are so far developed that they are capable of utilising the energy of the sun in the performance of their functions, they take up the horizontal position.

Another interesting matter for spring observation is the relative times of the bursting of the flowering buds and the leafing buds on the same species of tree or shrub. In many cases the former are fully developed before the latter show any signs of active growth, or while the foliage is as yet only passing through its earliest stages. The Hazel catkins shed their abundance of pollen before the foliage buds show the slightest signs of green. The Blackthorn is white with snowy blossoms before a leaf appears. The upper twigs of the Elm appear fluffy in the distance through the formation of its flowers while the foliage buds are still dormant; and the Alder, Willow, Poplar and Aspen likewise produce full-blown catkins while their branches are otherwise bare. Of the trees above named, the Hazel, Elm, Alder, Poplar, and Aspen are dependent on the spring winds for the transfer of the pollen, but the pollination of the Willow and the Blackthorn is brought about by the agency of early insects which visit the flowers for the nectar they provide.

Seedling of the Beech, Showing the Cotyledons and the First Foliage Leaves.