CHAP. i.J PLANT ARCHITECTURE. 11 



dividing the cell into two equal parts ; after the dividing 

 wall is formed it begins at the circumference to split 

 into two parts which will show under the microscope as 

 a minute notch at each end of the dark line in the cir- 

 cumference of the cell; this splitting becomes deeper 

 and deeper, and finally the two halves completely sepa- 

 rate from each other and form two new individuals. 

 Each young Pleurococcus is at first hemispherical, but 

 within a very short time becomes spherical and enters 

 the vegetative phase until it reaches the full size, when 

 it divides in a similar manner to its parent, producing 

 in turn two young plants. Reproduction by the splitting 

 up of an entire individual into two young plants is a 

 purely vegetative method, that is, not the result of ferti- 

 lization, and is termed, reproduction by fission or cell- 

 division, and is the most usual method amongst the 

 lower forms of life, both plant and animal. When the 

 Pleurococcus is placed under favourable circumstances, 

 reproduction often takes place so rapidly that when the 

 plant has formed the first wall for division into two 

 individuals, each half commences to divide again by a 

 wall at right angles to the first ; such individuals show 

 two lines crossing each other at right angles, and with 

 constrictions at the margin corresponding to the amount 

 of separation of the four individuals that has taken place. 

 The Pleurococcus can only obtain food and live actively 

 when a certain amount of moisture is present, conse- 

 quently during very dry weather it becomes dormant 

 and forms a dry, crumbling powder that is blown about 

 as dust. In this dry, powdery condition, the plant pos- 

 sesses the power of retaining life for a long time, even 

 many years, and when placed in a damp situation, at 



