168 



SCIENCE 



[N. S. Vol. XXXIII. No. 840 



has shown that if cactuses are cultivated in 

 darkness, their form changes completely. The 

 young shoots are rounded, and fail to show the 

 angular irregularities of form which increase the 

 surface capable of effecting assimilation under 

 the influence of light. 



Since the leaves especially are effective 

 in the light, the absence of light will pre- 

 vent normal development of the leaves and 

 this seems to be the chief direct effect. If 

 the plant does not have to develop leaves 

 to any extent, there is more food available 

 for the stem; and the growth of the stem 

 is thus really a secondary effect. The mat- 

 ter is still further complicated by the fact 

 that the moisture content and the carbon 

 dioxide content of the air were not kept 

 constant during the two sets of experi- 

 ments. 



An increase in the intensity of the light 

 is often accompanied by a decrease in the 

 surface of the leaf and an increase in the 

 thickness. ^^ I think that it is a mistake to 

 attribute this change to the action of light 

 alone. It is more the combined effect of 

 light and dryness, or of light and the 

 higher rate of evaporation due to a higher 

 temperature. "With decreased surface and 

 greater thickness there is less evaporation, 

 one extreme of which is reached in the 

 eucalyptus with its leaves turned edgewise. 

 The assimilation of food is provided for by 

 an increased thickness of the chlorophyll 

 layer, because the more intense light can 

 penetrate farther into the leaf. 



We have seen that the pigment in the 

 negro's skin is essentially protective in ac- 

 tion. A thicker or coarser leaf may also be 

 a protection against too intense a light. 

 Eowlee^* found that "intense light does 

 not kill thick, coriaceous or succulent 

 leaves with heavy cutinized external walls 



"^ Vernon, " Variation in Animals and Plants," 

 248, 1903. 



"Proc. 19th Meeting See. Promotion Agric. 

 Science, Boston, 1898. 



. . . doubtless owing to the screening effect 

 of the heavy walls or cells containing much 

 water. ' ' 



We have not sufficient data to make it 

 possible to say why an increase in the in- 

 tensity of light causes the change from sex- 

 ual to asexual reproduction in some algee 

 and the reverse change in others ;^^ but we 

 can be quite certain that both changes are 

 in conformity with the theorem of Le 

 Chatelier. 



MOISTURE 



Vernon^" says that "the effect of a dry 

 soil and atmosphere is well shown by the 

 characters of desert plants. These are 

 stunted in growth, and are of a nearly uni- 

 form gray color, owing to their intense 

 hairiness. The leaves are more fleshy, and 

 there is a great tendency to the formation 

 of spines. That these characters are in 

 part at least the direct result of want of 

 water is shown by the fact that they may 

 disappear if water is supplied." The de- 

 velopment of hairs is of great advantage to 

 a plant in an arid climate, especially if 

 there is any wind. The circulation of air, 

 and consequently the rate of evaporation 

 is impeded by the mass of hairs. 



We get characteristic changes when 

 plants, normally terrestrial, are grown in 

 water.^^ 



As regards the leaves, it is well known that 

 when aerial and floating leaves are present on 

 the same aquatic plant, they differ greatly in 

 structure, and as a rule also in form, from the 

 submerged leaves. In Ranunculus heterophyllus 

 and Cahomha aquatiea, for instance, the floating 

 leaves are more or less rounded, whilst the sub- 

 merged ones have dissected and filiform segments. 

 In Eippuris (mare's tail) the aerial and floating 

 leaves are short, and in CalUtriche rounded, but 



■^ Loeb, " Darwin and Modern Science," 230, 

 1909. 



^° " Variation in Animals and Plants," 263, 

 1903. 



" Cf. Vernon, " Variation in Animals and 

 Plants," 266, 1903. 



