6 OUTLINE OF PLANT GEOGRAPHY 



We may assume that the ancestors of the existing vegetation 

 of the earth were very simple fresh-water algae. 



In course of time there were manifested two divergent lines of 

 evolution in the plant kingdom, connected directly with a change 

 in environment. 



As the seas became more and more saline, we may suppose that 

 certain of the primitive algae adapted themselves to the denser 

 salt water, while others confined themselves to the fresh water 

 streams and lakes. At the present day two classes of algae, the 

 brown and the red, are preeminently the marine plant types, 

 aside from certain minute floating forms like the diatoms. Ex- 

 cept for a relative small number of green algae, and a few flowering 

 plants like the eel-grass, etc., the marine coastal flora is mainly 

 composed of the red and brown sea-weeds. 



The largest class of algae is that of the diatoms, unicellular 

 plants of which over 10,000 species are known. Owing to their 

 characteristic flinty shells, they are found in a fossil state in enor- 

 mous quantities. In spite of their simple cell-structure, which 

 would imply that they are primitive forms, the fossil record in- 

 dicates that they are among the most recent of plant types, no 

 certain remains being known much below the Cretaceous. As 

 they are especially abundant at present in cold waters, being 

 particularly numerous in the arctic and antarctic seas, it is 

 possible that they are forms which are especially fitted to cold- 

 water conditions and owe their great development to the refrigera- 

 tion of the ocean, and the general cooling of the earth's climate 

 which developed during the Tertiary, culminating in the Pleisto- 

 cene glaciation. 



Much more significant was the abandonment of the primeval 

 aquatic habitat for life on land. This was undoubtedly the most 

 momentous event in the history of the vegetable kingdom. The 

 much greater range of conditions on land, involving questions of 

 water storage and the development of mechanical or supporting 

 tissues, as well as adaptation to greater range of temperature 

 and other conditions, at once open up a practically unlimited field 

 for the operation of natural selection, and marked the beginning 

 of the reign of land plants which henceforth were to dominate 

 the vegetation of the world. 



Just when the first algae left the water and took up their abode 



