Anatomy and Histology. 125 



THE LEAF. 

 COTYLEDONS. 



The cotyledons of the guayule are dorsiventral (plate 34, figs. 7 to 9). 

 Both surfaces are free from trichomes. The parenchyma is composed of 

 six layers of cells, of which the upper two form a palisade tissue. The 

 spongy parenchyma is not highly differentiated, for the two lower layers 

 of cells only have distinctive characters, and these are not pronounced. 1 

 There are no resin-canals in the blade, though the four primary canals of 

 the hypocotyl pass, two into each of the petioles. The mid-vein (plate 

 3 1 , fig. 1) in the petiole is composed of two mestome strands, the origin of 

 which has already been discussed, but is single above, and there are two 

 lateral veins. The cotyledons show certain well-marked responses to 

 water and light conditions. The cotyledons of seedlings grown in the 

 shade are slightly thinner than those of field seedlings, and have a much 

 larger superficies (plate 34, figs. 6, 9). All the parenchyma cells, and all 

 the epidermal cells save the guard-cells, are expanded in directions parallel 

 to the surface. The epidermal cells of the surface are deeper also. The 

 intercellular spaces of the spongy parenchyma are more extensive, and 

 the cell-walls are thinner. These changes are in accord with observations 

 in general, but it is of importance to note that the internal and external 

 adjustments of the cotyledon are produced by changes in shape of the 

 cells, and not by change in number of cells. This is well illustrated by 

 the behavior of the epidermis, both as to the shape of the cells and the 

 number of stomata. One might well suppose that there would be an 

 increase in the number of stomata, as well as in their size, in plants well 

 supplied with water and not subjected to severe aerial conditions. Counts 

 of the stomata per unit of surface gave the results shown in table 50. 



That is, the number of stomata per unit of area appears to depend on 

 the amount of growth of the leaf. The greater number on the lower 

 surface in the field plant is due to the rolled-leaf effect, which is absent 

 from the shade plant. The result is that, in the plant which has to con- 

 serve water, there are relatively more stomata to carry it off. 2 Evidently 

 therefore, the supposedly adaptive adjust- 

 ments as regards the stomata do not involve 

 their numbers in plants of the same species 

 under different conditions. The thinner and 

 less strongly cuticularized epidermis of the 

 shade plant may indeed compensate, as may 

 also the more extensive intercellular system, 

 for the relatively fewer stomata. But inasmuch as the dampering of trans- 

 piration by stomata is not effective within wide limits (Lloyd, 1 908a) , such 

 differences in numbers as the above may be of little or no significance. 3 



1 The structure is very similar to that of the cotyledon of Helianthus, in 

 which, however, the intercellular spaces are relatively more extensive, and there 

 are more layers of cells. 



2 Transpiration rate is greater per unit of surface in sun plants than in shade 

 plants (Bergen, 1908; Sampson & Allen, 1909). 



3 On this question the student should consult Renner, 19 10. 



