THALLOPHYTES 



15 



FIG. 6. Coleochcete: a 

 flat, plate-like body; 

 from such an alga 

 body the land plants 

 were probably de- 

 rived. 



In the first place, the cells composing the individual may 

 divide freely in every plane, and this results in a massive body. 

 This form is not very common among 

 Algae, because it is not the most favor- 

 able arrangement of cells for free ex- 

 posure to water. 



In the second place, the cells com- 

 posing the individual may divide chiefly 

 in two planes, at right angles to one 

 another, and this results in a flat, plate- 

 like body, which may be a single layer 

 of cells in thickness, or several layers 

 (Fig. 6). This form of body is more 

 favorable for water exposure than the massive form, but 

 it is not the most favorable. 



In the third place, 

 the cells composing the 

 individual may divide 

 only or chiefly in one 

 plane, or rather in a 

 series of parallel planes, 

 and this results in a fil- 

 amentous body (Figs. 

 7, A, and 8). This is 

 by far the most common 

 form of body among 

 the Algae, especially the 

 fresh-water forms, and 

 it permits not only 

 every cell to come into 

 contact with water 

 freely, but also the free 

 swaying movements 

 that are of advantage in water. 



Another tendency in many-celled plants, which results in 



FIG. 7. Ulothrix: A, base of filament, showing 

 the holdfast cell and five of the ordinary cells 

 above ; B, four cells of a filament containing 

 spores ; C. showing one cell (a) containing 

 four swimming spores, a free swimming spore 

 (6), four escaped gametes (c), pairing gametes 

 (d), and two oospores (e) each of which has 

 been produced by the fusion of two gametes ; 

 Z>, a young filament started by a swimming 

 spore. 



