30 



BOTANY 



PART 1 



direction of growth of the primary shoot. Sometimes a main axis 

 is indistinguishable among the group of similarly directed branches. 

 The general appearance of the plant is determined by the direction 

 and thickness of its branches and leafy twigs. If these are all 

 directed upwards the shape is pyramidal, while broadly pyramidal, 

 oval, and rounded forms arise when the branches diverge more 

 strongly. The " weeping varieties " of several familiar trees are due 

 to the branches becoming long and pendulous. Herbaceous plants 

 often have stems which creep on the surface of the ground. Other 

 plants, both herbaceous and woody, climb upon various supports by 

 means of hooked hairs, prickles, and modified shoots, by means of 

 roots or tendrils, or by twining movements. Climbing plants are 

 termed LIANES, those which wind round a support being distinguished 

 as TWINING PLANTS. It is the presence of numerous rope-like stems 

 of lianes which renders the tropical forest so impenetrable. On 

 creeping shoots the leaves tend to become displaced towards the 

 dorsal surface of the stem, while branches spring from the sides and 

 roots from the ventral surface. 



In catalogues and descriptions of plants the duration of the period of growth 

 is usually expressed by special symbols : thus indicates an annual ; a biennial, 

 and 11 a perennial herb ; fj is employed to designate both trees and shrubs, and 

 for trees the sign ^ is also in use. 



Development of the Leaf. — The first appearance of the leaf as a 

 lateral protuberance (Fig. 17/) on the vegetative cone of the shoot 



has already been referred to (p. 19). AVhen 

 the apex of a shoot is removed by a transverse 

 section and viewed from above (Fig. 31), the 

 origin of leaves as lateral protuberances is still 

 more evident than in a longitudinal section. The 

 embryonic leaf rudiment generally occupies l)ut 

 a small portion of the periphery of the vegeta- 

 tive cone ; it may, however, completely surround 

 it. In like manner, when the mature leaves 

 are arranged in whorls, the developing protuber- 

 ances of the rudimentary leaves may foi'm at 

 first a continuous wall-like ring around the 

 F.a. 3i.-Ai,icai view of the growiug point and Only give rise later to the 

 vegetative cone of a shoot separate leaf rudiments. Leaves take their origin 

 of Euonymus iaponicus. ^^jy fj.Qjjj svxch. parts of a plant as have remained 



in an embrj'onic condition. A leaf never arises 

 directly from the older parts of a plant. In cases where it apparently 

 does so its development has been preceded by the formation of a 

 growing point of a new shoot. The grooving point of a shoot has 

 usually an unlimitkd GROWTH, while the growth of a leaf is limited. 

 A leaf usually continues to grow at its apex for a short time only, 



