lyo THE STRUCTURE OF ECONOMIC PLANTS 



ately within the epidermis. Those adjacent to the smaller bundles 

 consist of a few fibers; but near the larger ones, the fibrous tissue is 

 much more extensive. It may occupy the entire space between the 

 bundle and the epidermal layers, so that a continuous band of 

 sclerenchyma is formed, consisting of the fiber strands and the 

 mechanical tissue of the inner layer of the bundle sheath. In 

 the leaf sheath, the sclerenchyma is usually restricted to zones 

 adjacent to the abaxial surface, the amount increasing toward the 

 node; but there are a few mechanical elements abutting the 

 adaxial one, except opposite the smaller bundles. 



The ligule is a non-vascular emergence which arises at the 

 junction of the blade and sheath. It is parenchymatous and is 

 three or four cells in thickness at its base. The inner surface 

 of the ligule is continuous with the epidermis of the leaf sheath, 

 and both surfaces are without hairs or stomata. The free upper 

 edge may be somewhat fringed, due to the elongation of its mar- 

 ginal cells. 



The Ontogeny of the Shoot. — The development of the grow- 

 ing point of the stem axis, and the differentiation of the leaves and 

 axillary buds, have been investigated by Rosier (ix). The periph- 

 eral layer of the bud meristem constitutes a dermatogen which 

 divides only anticlinally, except at points where a leaf primordium 

 is initiated. The leaf originates entirely from the dermatogen, 

 the first evidence of primordial development being the periclinal 

 division of a group of these cells . Rapid divisions of the derivative 

 cells follow, and there is an early blocking off of the mesophyll 

 region. The cells of the subepidermal layer, underlying the leaf 

 primordium, also divide tangentially; but they contribute only 

 to the development of the vascular system at the base of the leaf 

 and the internode. 



The formation of new buds or growing points involves the 

 subepidermal tissues; and, according to Rosier, two different 

 types of ontogeny may occur. In young plants, the tissue of the 

 bud within the dermatogen may arise from a single subepidermal 

 apical cell; or in some cases, from a plate of several subepidermal 

 cells. This initial, or group of initials, forms the corpus of the 

 axis, and by periclinal divisions two layers are produced. These 

 are not exactly comparable to Hanstein's periblem and plerome, 

 since the outer one gives rise to the cortex and the peripheral 

 part of the stele, while the inner one differentiates the remainder 



