20 



BOTANY. 



Of the forms that are united in colonies 1 one of the best 

 known is Volvox (Fig. 10). This plant is sometimes found in 

 quiet water, where it floats on or near the surface as a dark 

 green ball, just large enough to be seen with the naked eye. 

 They may be kept for some time in aquaria, and will some- 

 times multiply rapidly, but are very susceptible to extremes 

 of temperature, especially of heat. 



The colony (Fig. 10, A) is a hollow sphere, the numerous green cells of 

 which it is composed forming a single layer on the outside. By killing 



with iodine, and using a strong lens, 

 each cell is seen to be somewhat pear- 

 shaped (Fig. JB), with the pointed end 

 out. Attached to this end are two 

 vibratile filaments (cilia or flagella), 

 and the united movements of these 

 cause the rolling motion of the whole 

 colony. Usually a number of young 

 colonies (Fig. x) are found within the 

 mother colony. These arise by the 

 repeated bipartition of a single cell, 

 and escape finally, forming independent 

 colonies. 



Another (sexual) form of reproduction occurs, similar to that found in 

 many higher plants ; but as it only occurs at certain seasons, it is not 

 likely to be met with by the student. 



Other forms related to Volvox, and sometimes met with, are 

 Gonium, in which there are sixteen cells, forming a flat square ; 

 Pandorina and Eudorina, with sixteen cells, forming an oval or 

 globular colony like Volvox, but much smaller. In all of these 

 the structure of the cells is essentially as in Volvox. 



1 The term "colony" is, perhaps, inappropriate, as the whole mass of 

 cells arises from a single one, and may properly be looked upon as an 

 individual plant. 



FIG. 10. Volvox. A, mature col- 

 ony, containing several smaller 

 ones (x), x 50. B, Two cells 

 showing the cilia, x 300. 



