LINUM USITATISSIMUM 391 



and adaxial portions of the primordium results in the formation of 

 the lamina. As this proceeds, the cells of the mesophyll undergo 

 enlargement, and the procambial strands, which were determined 

 early in ontogeny, differentiate as vascular bundles. 



The meristematic activity does not cease at one time throughout 

 the developing leaf, but is discontinued first at the tip of the leaf, 

 then at its base, and finally in the midbasal portion of the lamina. 

 (Fig. 1.00, A, B, C.) Following the cessation of cell division, 

 which occurs when the leaf has attained about one-fifth its mature 

 size, further enlargement is accomplished by the expansion, stretch- 

 ing, and separation of the cells already formed, and, at maturity, 

 the mesophyll is very loose and spongy with large intercellular 

 spaces. (Fig. 1.00, D.) It consists of about four cell layers which 

 are not differentiated into a well-defined palisade and spongy paren- 

 chyma, and the cells are all somewhat elongated and essentially 

 alike. A network of veins extends throughout the mesophyll, 

 forming a complicated system made up of bundles of various sizes. 

 The midvein and occasionally the two larger lateral veins may 

 develop some secondary tissue. The epidermal cells are irregular 

 in size with a thin cuticle, and numerous stomata occur in about 

 equal numbers on both surfaces of the leaf. The guard cells 

 are subtended by accessory cells in which, as pointed out by 

 Haberlandt (9), the thin walls act as hinges in the stomatal 

 mechanism. 



The successive leaf primordia are at first very close to one another 

 so that there may be ten to twenty nodes and internodes in a grow- 

 ing point which is only o.i to 0.15 mm. in length. The provascular 

 tissue of the epicotyl appears to be differentiated simultaneously 

 in the leaf primordia and meristematic tissue of the growing point, 

 and each strand extends downward through the axis for several 

 internodes, keeping pace with internodal elongation so that there 

 is no change in the fundamental plan of the vascular system as 

 differentiation proceeds. 



All the bundles of the mature vascular system are collateral and 

 common. The vascular supply to each leaf consists of three large 

 bundles which pass through the cortex separately, anastomosing 

 with one another at the point where they form a part of the stele 

 of the stem, and gradually becoming smaller at lower levels until 

 they end blindly after extending through fifteen to twenty nodes. 

 There are usually eight to ten foliar bundles which extend through 



