186 REPRODUCTION OF SPIROGYRA 



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 growth 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 Spirogym (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 

 motile 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 asexual 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 walls at the point of contact are absorbed, forming an 

 open passage between the two cells. While this growth has been 

 going on, the contents of the two cells have contracted some- 

 what and finally the contents of one cell passes through the tube 

 and fuses with the contents of the other cell. Several modi- 

 fications of the reproduction process outlined above will occa- 



