148 



THE VARIOUS FORMS OF PLANTS 



Careful study shows the presence of strands held in the body of 

 the cell by strands of protoplasm, the remainder of the space 

 within the cell being occupied by the cell sap. 



Pond scum may grow by a simple division of the cells in a fila- 

 ment. This method of asexual reproduction is the way growth 

 takes place in the cells of the root, stem, or leaf of a flowering 

 plant, but another method of reproduction is also seen in pond 

 scum. The cells of two adjoining filaments may push out tubes 

 which meet, thus connecting the cells with each 

 other. Meantime the protoplasm of the cells 

 thus joined condenses into two tiny spheres; the 

 bands of chlorophyll are broken down, and ulti- 

 mately the contents of one of the cells passes over 

 the tube and mingles with the cell of the neigh- 

 boring filament, with which it was previously con- 

 nected by the tube formed from the cell walls. 

 The result of this process of fusion is a thick- 

 walled resting cell which we call a zygospore. 



Conjugation. — The process in which two cells 

 of equal size unite to form a single cell is called 

 conjugation. It is believed to be a sexual process 

 which corresponds in a way to the fertilization 

 in the higher plants.^ This cell thus formed can 

 ^^dthstand considerable extremes of heat and cold, 

 and may be dried to such an extent that it is 

 found in dust or in the air. Under favorable 

 conditions, this spore will germinate and 

 produce a filament. 



Pleurococcus. — Many other forms of algsB 

 are well known to us. One of the simplest is -^s^is' r> 



pleurococcus. This little plant consists of a -di ^ . i h 



• , ^- ,, , . , , ,. . . . FleuTococcus. A, single cell; 



single tiny cell, which by division may give ^ .^lony of four cells 



rise to two, three, four, or even more cells formed from the original 



which cling together in a mass. The green cell A. 



Conjugation of 

 Spirogyra; zs, 

 zygospore; /, 

 fusion in progress. 



' Material which shows conjugation is not always easy to obtain. Conjugation 

 usually takes place most freely in the fall of the year. When material is obtained, 

 it may be preserved in a 4 per cent solution of formol. Material killed in a 5 per 

 cent solution of chromic acid and then preserved in 70 per cent alcohol or 4 per 

 cent formol shows the details of ceUular structure. 



