DEVELOPMENT OF THE LEAF. 155 



has been termed a fascicle. Comparison of these threads --if 

 such they can indeed be called - - with the rudimentary fibro- 

 vascular bundles of some water-plants suggests that the former 

 are bundles of the simplest possible kind. 



The parenchyma cells are bounded in true mosses by smaller, 

 thicker- walled cells, which do not contain chlorophyll. 



THE LEAF. 



433. It was shown in 322 that roots are formed under the 

 superficial tissues of the stem, and have these outer layers, or 

 derivatives from them, as coverings during at least a portion 

 of their growth. But leaves are never thus covered by layers of 

 stem-tissue ; hence they are termed exogenous productions, 

 while the term endogenous is applied to the manner in which 

 roots are formed. 



434. Development. In the earliest stage of its development 

 the leaf is a mere papilla consisting of nascent cortex (periblem) 

 and nascent epidermis (dermatogen). As soon as the papilla 

 elongates, or becomes flattened, some of its interior cells, making 

 up procambium tissue (see 315), differentiate into fibro-vascular 

 bundles. But the procambium of the nascent leaf and that of 

 the cone of soft tissue constituting the growing-point of the 

 stem are in unbroken connection with each other ; in like man- 

 ner the bundles which are derived therefrom are continuous, and 

 it is not possible to detect any line of demarcation between them. 

 In fact, the newly formed bundles in a young leaf appear as if 

 they are merely the slender prolongations and terminations of 

 those in the young stem. 1 



435. With the transverse and longitudinal enlargement of the 

 nascent leaf there is generally more or less curvature, so that 

 the outer, lower, and earlier leaves infold the upper leaves and 

 the growing-point of the cone. In most cases, some of the 

 lower leaves which thus envelop the growing-point become modi- 

 fied to form protecting scales ; such is the ordinary structure of 

 buds (see " Structural Botany," page 42, fig. 83). 



1 It should be remembered, however, that some of the bundles in the stem 

 (see 365) may be derived from procambium peculiar to the stem, and which 

 does not extend into the leaf. Hence it is necessary to distinguish between 

 stem-bundles, common bundles, and leaf-traces. The former belong to the 

 stem alone ; the common bundles are common to stem and leaf; the leaf-traces 

 are leaf-bundles which are in the stem and which at some point unite with 

 other bundles of the same kind to form common bundles. 



