THE EVOLUTION OF THE SPOROPHYTE. 3 



we find in most such algae a development of mucilaginous 

 tissue which prevents their complete desiccation. 



These seaweeds have adapted themselves perfectly to their 

 peculiar environment, and such highly specialized forms as the 

 great kelps and many red algae probably represent the highest 

 types of these marine plants. They have diverged widely 

 from the simpler fresh-water algae, and there is no reason to 

 suppose that they have given rise to any higher types. 



The simple fresh-water green algae, which, so far as we 

 know, most nearly represent the ancestors of the terrestrial 

 green plants, differ much in their conditions of life from the 

 seaweeds. Most bodies of fresh water are subject to great 

 fluctuations of depth, often drying up completely for long 

 periods, or sometimes being frozen. It is obvious that plants 

 living under such conditions must be very resistant, and we 

 find that such is the case among most green algae. Not only, 

 as a rule, are they capable of enduring a great range of tem- 

 perature, but usually at the end of their vegetative period they 

 produce special cells, "spores," which can endure complete 

 desiccation without injury, and are also uninjured by freezing. 

 By means of these " resting-spores " the plant is carried over 

 from one growing period to the next, and when the conditions 

 are favorable, the spores germinate and give rise to a new 

 generation. This production of resting-spores is one of the 

 most striking differences between these fresh-water algae and 

 their red and brown relations in the sea, where there is usually 

 no necessity for such resting-spores. 



Certain green algae, like some species of the common genus 

 Vaucheria, may be considered amphibious, as they do not 

 actually grow in the water, but are exposed to the air on the 

 surface of moist earth, from which they absorb the water 

 necessary for their growth. The ability to thus grow with a 

 diminished water supply is an evident advantage, and in some 

 such way as this it is probable that the first strictly terrestrial 

 plants originated from some originally aquatic algal ancestors. 



We can imagine some such forms gradually becoming better 

 and better able to vegetate on the mud left by the subsidence 

 of the water, and finally becoming adapted to life on land. The 



