7O Practical Plant Biology. 



anabolism for the assimilation of nitrogen and for photosynthesis. 

 Hence excretion becomes unnecessary. 



The accumulation of assimilated material is accommodated by 

 the growth and elongation of the cell. After continued elonga- 

 tion the nucleus divides. The two resulting nuclei remain con- 

 nected together by a barrel-shaped mass of cytoplasm occupying 

 the middle of the cell. At the same time an annular deposit of 

 cellulose may be observed forming on the inside of the cylindrical 

 surface of the cell-wall about midway between the two end walls. 

 This annular deposit increases until it forms a diaphragm partially 

 dividing the original cell into two. The barrel-shaped mass con- 

 necting the nuclei is ultimately encroached upon by the gradual 

 closing of the perforation in the diaphragm and the chloroplasts 

 are also cut in two by it. Finally the perforation closes entirely 

 and divides the original cell into two separate compartments each 

 having its own nucleus which now draws away from the partition 

 and occupies the middle of the new cell. The new cell may now 

 grow and the process be repeated. It is observed that cell and 

 nuclear division usually take place during the night or early hours 

 of the morning : but when once started they may be delayed by 

 exposure to a low temperature. 



Growth and cell division may take place in any cell and lead 

 to the rapid lengthening of the filaments. The adjoining cells 

 of the filaments are very slenderly attached to each other so 

 that the lengthened filaments easily break into parts. Thus 

 multiplication and reproduction is effected. This method of 

 reproduction is very prolific during the growing period of the 

 cell, viz. in spring and summer. 



Besides this asexual method, Spirogyra has a sexual method 

 of reproduction, to which it appears to resort at the end of a 

 period of vigorous growth. At this stage Spirogyra takes on a 

 very different appearance and also feels different to the touch. 

 It loses its brilliant colour and becomes gradually pale or even 

 brownish. Its filaments meet together and form a tangled mass. 

 The process, which is called conjugation, begins by each of the 

 opposite cells of two adjacent filaments pushing out a slight 

 prominence on the side nearest its opposite neighbour. The 

 protoplasm of the cells extends out into these outgrowths which 

 become tubular in form and are each directed toward the similar 

 outgrowth from the opposite cell. After close contact has been 

 established between the opposite pairs, the walls covering the 

 ends of the outgrowths are dissolved and the contents of the 

 two cells come into union with one another through this " con- 



