THE ANATOMY OF THE SHOOT 75 



occur on many conifers, and are also found on such trees as the 

 olive, orange, and others. Many perennials have a deciduous 

 habit, and a new set of foliage leaves is differentiated each year 

 which is shed at the end of the growing season, or at the beginning 

 of the ensuing period of vegetative extension. In annual plants, it 

 is not uncommon for the cotyledons and basal leaves to be shed 

 before the growth cycle is complete. In deciduous types, leaf fall 

 is commonly accomplished by means of an abscission layer, which is 

 usually located at the base of the petiole, but may sometimes be 

 developed at the base of the lamina or at an intermediate point in 

 the petiole. The separation of the leaf from the axis results from 

 the development of the cells of the abscission layer which round 

 off and separate from one another owing to the disintegration of 

 the intercellular substance of their middle lamellae. A detailed 

 account of this phenomenon is given for Lycopersicum. (See 

 Chapter XVIII.) 



ANATOMY OF THE LEAF 



The leaf may include all the types of tissues which are differenti- 

 ated in the stem (epidermis, parenchyma, mechanical, and vascular 

 tissues). As might be expected on the basis of the ontogeny of the 

 leaf, these tissues are continuous with those of the stem axis. 

 Thus, the epidermis of the adaxial and abaxial surfaces of a foliage 

 leaf forms an uninterrupted protective layer with the epidermis 

 of the stem; the vascular bundles of the leaf are continuous with 

 those of the stem axis at the node, and the parenchymatous regions 

 of the petiole are contiguous with parenchyma of the stem. 



Structurally, the leaf consists of three major regions: (i) the 

 protective epidermis, which covers its entire surface, (i) the meso- 

 phyll, which is generally parenchymatous, and (3) the veins. 

 (Fig. i8.) The character of the epidermis, the specialized stomatal 

 apparatus, and the types of epidermal hairs which may be produced 

 by this tissue have been discussed. (Chapter I.) The epidermis 

 may be regarded as consisting in most cases of an impervious, non- 

 chlorophyllose (except for the guard cells) layer which may or 

 may not develop a non-cellular cuticle. Stomata occur in varying 

 frequencies and distributions, so that ready means of gas exchange 

 is provided. The number of stomata on the abaxial surface usually 

 exceeds that on the adaxial one when leaves are oriented dorsi- 

 ventrally. In those that assume a more or less vertical position 



