8z THE STRUCTURE OF ECONOMIC PLANTS 



In some cases, it has been shown that the development of the blade 

 results from the activity of a band of marginal cells which are 

 located along the edge of the midrib; while, in others, additional 

 underlying layers, as well as the subepidermal ones, are involved 

 in the development of the lamina. 



The characteristic thinness of the blade of foliage leaves can 

 be related to its ontogeny. There is a relatively constant number 

 of layers of meristematic cells in the embryonic blade, ranging from 

 five to eight; and, from these layers, the epidermis, mesophyll, and 

 vascular tissues of the mature blade are differentiated. Examples 

 of the meristematic layering in the young lamina include the bean 

 and sweet potato with six layers, and tobacco with six or seven. 

 From these layers, the mature tissues of the leaf are derived in a 

 definite cellular succession which is essentially constant for a given 

 species. 



In general, the upper and lower layers function as dermatogen, 

 producing the upper and the lower epidermis respectively. The 

 adaxial subepidermal layer develops the palisade parenchyma; the 

 middle layer or layers produce the provascular strands, and ulti- 

 mately the vascular tissue; and the abaxial subepidermal layer 

 produces the major portion of the spongy parenchyma. Modifica- 

 tions in this sequence of cell lineage occur, depending upon the 

 number of initial cell layers in the embryonic blade. In some cases, 

 derivatives of the middle layers form a part of the spongy paren- 

 chyma, in addition to producing the vascular system; and, where 

 a double palisade occurs, they may form a portion of that 

 region. 



The specificity of the initial cell layers of the young leaf with 

 respect to the tissues ultimately derived from them has long 

 been recognized and three generative layers corresponding to the 

 three histogens of the stem were outlined by Flot (14) and Gravis 

 (ii). On this basis, the formative layers of the lamina were 

 designated by Schiiepp (36) as protoderm, giving rise to epidermis; 

 ground meristem, producing the parenchymatous tissues of the meso- 

 phyll; and procambium, from which the vascular tissues are formed. 

 The growth of the blade in surface area is due to the activity of 

 what he has termed plate meristems. This activity follows the mar- 

 ginal growth of the lamina and continues long after the latter has 

 ceased. Marginal growth is not discontinued simultaneously in all 

 parts of the leaf; and, in the basipetal type of development, the 



