DEVELOPMENT OF PLANTS 175 



as pond scum or frog spittle. They can usually be recognized 

 by their slimy nature which is due to the copious exudation of a 

 gelatinous substance from the cell walls. In one group of this 

 order the plants are multi-cellular (Fig. 106), the cells being at- 

 tached end to end and forming a filament — though it should be 

 stated that each cell is practically independent in its life process 

 and the organism may therefore be regarded as a colony. Such 

 a type of plant would have originated in the case of Pleurococcus 

 if the two daughter cells had remained attached and continued 

 to divide in but one plane and parallel to the first division wall. 

 This is exactly the mode of grow^th in these filamentous plants. 

 Any of the cells may divide into two daughter cells which grow 

 to the original size of the mother cells. In this way the length 

 of the plant is increased. The most characteristic feature about 

 the Zygnematales is the remarkably large and attractive chloro- 

 plasts. In Spirogyra (Fig. 106, A) these appear as bands, from 

 one to several in each cell, spirally arranged just within the cell 

 wall. In Zygnema (Fig. 106, B) they assume the form of stars 

 and in other genera they appear as plates, bars and variously 

 modified bodies. Imbedded in the plastids are denser proto- 

 plasmic bodies, the pyrenoids, that appear to be connected with 

 the secretion of starch ; at least a layer of starch can be detected 

 about the pyrenoids by testing with iodine (Fig. 106, pr). 



The filaments are fragile and readily become dissociated into 

 smaller portions. In this way, the plants may multiply since the 

 fragments continue to grow after the manner of the original 

 filament. Singularly, there is no indication of a return to the mo- 

 tile condition in the life history of these plants. No zoospores or 

 motile gametes are formed. Sometimes the walls of a cell become 

 thickened and after a dormant period, as through the winter, may 

 germinate and grow into a new plant. With this rather rare 

 exception, the asexal reproduction of the plant by spOres does 

 not occur. The sexual reproduction is of a peculiar character. 

 This is very well illustrated in Spirogyra. When filaments of 

 this plant chance to lie side by side, under certain conditions, 

 tube-like processes from opposite cells begin to grow out towards 

 each other (Fig. 107, A). These tubes finally meet, when the 



