364 STRUCTURE AND LIFE HISTORIES 



So the differentiation of the plant body into root, stem, 

 leaves, and organs of reproduction means greater efficiency 

 in the performance of all the functions of life. 



329. From Water to Land. A careful consideration of 

 all available evidence leads to the conviction that plant 

 life originated in the water. For example, the more 

 primitive types of plants have no well-defined polarity, 

 that is, they do not present an axis with the opposite ends 

 clearly differentiated for the performance of different 

 functions under unlike surroundings, such as roots ad- 

 justed to a life in the soil and darkness, and leaves 

 adjusted to a life in the air and light. Plants submerged 

 in water, such as Spirogyra, commonly possess a uni- 

 formity of structure, in harmony with their uniform 

 environment. Of course, there are exceptions to this, 

 Ascophyllum^ Dictyota, Vaucheria, and other submerged 

 aquatics possess, on one end, hold-fasts which anchor them 

 to the substratum; but these plants probably represent 

 early steps toward a rooted existence on land. 



One of the most marked evidences of aquatic life for 

 primitive organisms is their method of reproduction by 

 motile spores and motile gametes; while, at the same time, 

 one of the most distinctive characteristics of the more 

 highly developed land-plants is reproduction by non- 

 motile spores, suited to distribution by wind. The 

 enormous number of spores produced insures a rapid 

 multiplication of individuals, and their dryness insures 

 protection during periods of more or less prolonged 

 drought. 



330. Development of the Sporophyte. For the suc- 

 cessful production of large numbers of spores there is 

 needed some provision for richly nourishing the spore- 



