Factors in Evolution 43 



the question of water. It is not always easy to dis- 

 tinguish between modifications due to light and 

 those concerned with the water supply. Thin, broad 

 shade leaves are correlated with the necessity of a 

 greater amount of chlorophyll tissue, owing to the 

 less powerful illumination, and also with the les- 

 sened evaporation due to the moisture in the air and 

 lower temperature of the deep shade. Many of the 

 lower plants, such as the algae and mosses which 

 are not able to survive complete drying up, but 

 which must provide against any such contingency, 

 have evolved reproductive cells spores which are 

 fitted to resist prolonged desiccation. In the seed- 

 plants there have also been developed special struc- 

 tures seeds, bulbs, tubers, etc. which enable these 

 plants to pass unharmed through periods of drought 

 or cold which are fatal to the plant in its active 

 condition. These reproductive structures absorb 

 water promptly and begin to grow when proper con- 

 ditions are presented. 



Formative Effects of Gravity. The continually 

 acting force of gravity undoubtedly exercises a 

 powerful formative effect upon all plant structures, 

 but the nature of this influence is still very obscure. 

 As a rule aerial shoots grow upwards, and are said 

 to be negatively geotropic, while roots growing 

 downward are positively geotropic. A young seed- 

 ling placed horizontally will quickly show an up- 

 ward curvature of the shoot and a downward bend- 

 ing of the root. Should the force of gravity be 



