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, Dictyoia, Vauckeria, 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- 
