126 Guayule. 



That the structural adjustments of the cotyledon involve only change 

 in shape of the cells is shown also by the responses of seedlings grown in 

 a soil of high osmotic equivalent (plate 34, figs. 5,8). Under this condi- 

 tion the surface of the cotyledon is greater than in the field plant (plate 34, 

 figs. 4, 7), but its thickness is also much greater. The cells of the paren- 

 chyma are correspondingly deeper (plate 34, fig. 8), their extension of size 

 being at right angles to the leaf-surface and parallel to the direction of 

 greater extension of its size. This cotyledon represents the maximum 

 response in a xerophytic direction, and it is worthy of note that under 

 normal field conditions this response does not ensue, indicating that 

 succulency here is primarily the effect of a soil condition, namely, the low 

 physiological availability of the soil- water. 



PROPHYLLS. 



The earliest foliage leaves (prophylls) show a slight advance toward 

 the bifacial condition, though normally they are dorsiventral. Neverthe- 

 less it is possible to induce a marked bifacial condition by growing seed- 

 lings in soil which contains a very meager supply of water (plate 35, figs. 5, 

 8). Such plants grow very slowly indeed, and the earliest foliage leaves 

 attain but a small size and are relatively thick, while the resin-canals are 

 of greater diameter. The extreme departure from this condition is shown 

 by shade-grown leaves (plate 35, figs. 1, 2), with the greater superficial ex- 

 tent of which the shape and dimensions of their cells are correlated, while 

 field seedlings and those grown in soil of high osmotic equivalent are very 

 similar in structure. To be noted, however, both in these leaves and in 

 the cotyledons, is the behavior of the spongy parenchyma. The lower- 

 most layer of cells shows this especially. In shade plants (plate 35, fig. 1) 

 the cells are broad, as viewed in a transverse section, and dumbbell- 

 shaped. In the field plant (plate 35, figs. 3, 4) they are columnar and 

 have two spaces between each two cells. In the seedlings exposed to dis- 

 tinctly unfavorable soil these spaces are almost, or frequently entirely, 

 absent (plate 35, figs. 6, 7). 



THE DEFINITIVE LEAF. 



Although the foliage leaves exhibit a structural advance over the 

 cotyledons, it is noticeable that, as compared with these, the definitive 

 foliage leaves exhibit a smaller range of response. These have, to be sure, 

 a dense clothing of trichomes, described elsewhere, and this fact may 

 explain the difference, which receives no elucidation in the character of 

 the stomata (plate 35, fig. 11). These show no special so-called adaptive 

 features. There can be no doubt that closely packed hairs form an effective 

 insulation which has the effect of producing mesophytic conditions, so to 

 speak, over the leaf-surface, both by dampering transpiration and by 

 modifying the sunlight. 1 



To determine the extremes of structural response within the leaf, I 

 have taken one leaf from an irrigated plant during the period of rapid 

 growth and one from a large seedling of good size after a six months' 



1 Cf. Wiegand's interesting paper of 1910. 



