96 Guayule. 



The leptome of the root does not show any starch-content in the 

 sieve part, though it occurs apparently erratically in its parenchyma and 

 in the cortical cells adjacent also to the leptome and to the resin-canals. 

 It is also to be found in the endodermal cells close to the primary canals, 

 and occasionally elsewhere, in a 4 mm. diameter root of a field plant. 



If, however, we examine a plant grown with an abundance of water, 

 in which the secretion of rubber has taken place only in minute quantities 

 and this in the wall-cells of the resin-canals, an important physiological 

 relation between starch and the secretory canal-cells is indicated. In a 

 root 4 mm. in diameter, which developed in about three months, the dis- 

 tribution of starch and its quantity are very striking. It is present abun- 

 dantly (a) in a broad, irregular radial band of cortical cells extending 

 from the primary resin-canals, (b) in a narrow and somewhat irregular 

 circular band midway the secondary cortex, and (c) in marked quantities 

 in the cortical cells adjacent to the definitive resin-canals. It is not pres- 

 ent in the leptome adjacent to the young resin-canals. It would therefore 

 seem probable that the presence of starch in marked quantities near the 

 resin-canals is related either to the secretion of rubber by the wall-cells 

 especially, or to the secretion of resin. The familiar case of Pinus, in 

 which starch occurs near the resin-canals, suggests the latter. 



The earliest appearance of rubber, which is secreted by the paren- 

 chyma of the cortical rays and of the cortex, aside from the cells of the 

 resin-canals as above noted, is to be seen in the innermost cells of the rays, 

 and synchronously in the outermost cells of the primary cortex, or, if that 

 is absent before secretion begins, of the secondary cortex. This fact is 

 beautifully shown in the tap-root of a seedling from the field, probably 

 less than one year old, collected on July 14, 1908, and measuring 2 mm. 

 in diameter. In this specimen the cells of the primary cortex were com- 

 pletely filled, as also the outer cells of the secondary cortex, there being 

 progressively less and less secretion toward the center of the root. The 

 opposite relation was shown by the parenchyma rays, in the cells of which 

 the amounts of rubber were found to be progressively less and less, as one 

 proceeded from the center outward (plate 23, figs. 3,7; plate 40, figs. 2 to 

 4). In a still younger seedling, perhaps three months old, about 1.2 mm. 

 in diameter, rubber is to be seen only in the cortical cells adjacent to the 

 primary canals and in the few innermost cells of the cortical rays. The 

 amount is so small here that, while it may readily be seen with the eye, 

 the photograph does not differentiate it. 



HYPOCOTYL. 



PRIMARY STRUCTURE. 



The primary cortex consists of six layers of cells, including the 

 endodermis. The epidermis becomes rather strongly cuticularized and 

 many of the cells are papillate, or, more correctly speaking, form short, 

 round-ended trichomes, which are usually one-celled, though two-celled 

 trichomes are found in a few instances (plate 23 , fig. 9) . The angles of the 

 cortical cells adjacent to the inner faces of the epidermal cells are collen- 

 chymatized, but in deeper layers this character is not present. Chloro- 



