Environment and Adaptation 193 



ter supplied to the plant is constant, and provisions 

 for the conduction of water and for its conserva- 

 tion are unnecessary; hence the absence of these 

 in such submersed aquatics as most of the algae. 

 However, where the plant reaches a great size, as 

 in the kelps, conducting tissues for the transport 

 of assimilated material may be very perfectly 

 developed. While variations of light to which algae 

 are exposed are somewhat less extreme than is the 

 case in land plants, it is evident that the question 

 of light has been one of the most important factors 

 in the modifications of the algal types, since the 

 varying depth of the water, as well as the shade 

 of the rocks and larger algae, must cause great dif- 

 ferences in the intensity of the light, with a corre- 

 sponding variation in the plants adapted to these 

 lights of different intensity. 



As we have endeavored to show, the first land 

 plants probably arose from forms allied to some 

 of the existing fresh-water algae, which became 

 adapted to life on land by the development of roots 

 for water absorption, and more or less perfect 

 protection of the exposed tissues against undue 

 loss of water. This is secured either by the de- 

 velopment of mucilaginous or gelatinous envelopes, 

 or by the cuticularization of the exposed cell walls. 

 This primitive type of land plant probably cul- 

 minated in the higher mosses, but it never became 

 quite perfectly adjusted to terrestrial conditions, 

 since the simple hair-like roots could only suffice 



