LYCOPERSICUM ESCULENTUM 575 



made microchemical and physical tests of the stem at different 

 developmental stages, and finds that the thickened angles of the 

 walls of the collenchymatous cells are composed of a large number 

 of fine cellulose lamellae which alternate with layers of a pectic 

 compound. The epidermal cells may develop to form either of 

 the two types of hairs noted for the flower and leaf. The stomatal 

 frequency is about half that found in the upper epidermis of the 

 leaf. 



The character and amount of secondary xylem formed is variable, 

 depending upon the degree of vegetativeness of the stem. As 

 Kraus and Kraybill (19) have pointed out, highly vegetative 

 plants grown with an abundant supply of available nitrogen have 

 stems that are much larger than those of feebly vegetative ones, 

 owing to the greater number and size of the pith cells. There 

 is a reduction in the amount of collenchyma and pith, accompanied 

 by a marked increase in the secondary xylem and a conspicuous 

 increase in the thickness of the walls of the pericyclic fibers in the 

 feebly vegetative stem. (Fig. 301.) 



The Leaf. — The blade of the leaflet is thin, the mesophyll 

 consisting of a single row of palisade cells and a very loosely 

 organized spongy region of four or five layers of chlorenchyma. 

 The major portion of the vein projects prominently on the lower 

 surface, and there is a band of abaxial collenchyma reinforcing it 

 which with the adaxial strand and the mechanical elements of the 

 bundle gives support to the leaflet. (Fig. 301, A.") The bicol- 

 lateral bundle is like that of the stem, and there may be some 

 cambial activity in the larger veins. In the smaller veins, the 

 xylem consists chiefly of spiral elements. Both adaxial and 

 abaxial phloem are present in the main laterals, but adaxial phloem 

 is not found in the smaller branches. The walls of the cells of the 

 upper epidermis are much less sinuous than those of the lower 

 surface, and the stomata and hairs are less abundant on the latter. 

 (Fig. 301, C, D.) The two types of hairs are similar to those 

 described for the floral parts and the stem. 



In the petiole, the epidermis is pubescent, and underlying it is 

 a continuous zone of chlorenchyma three or four cells in width. 

 Centrad to the chlorenchyma there is an uninterrupted band of 

 collenchyma which is three to six cells in width, and within this 

 is the parenchymatous tissue that surrounds the vascular strands. 

 The vascular strands are arranged in an approximate circle that is 



