634 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



function, and necessarily die after the reproductive cells have given rise to daughter- 

 colonies. This is the first time we have met with such natural death among the 

 Algse, and it is very cleai'ly seen to be connected with the separation of the assimila- 

 tive and reproductive functions. The formation of gametes in Pleodorina has 

 unfortunately not yet been observed. 



The remarkable and beautiful orffanism called Volvox has been known for more 



o 



than two centuries, and has long been among the most favourite of microscopic 

 objects. The purely scientific interest which it has aroused has been as great as 

 the SBsthetic admiration which it has excited. Long and animated controversies 

 have raged on the question as to whether it was to be regarded as an animal or a 

 plant, as an individual or as a colony of individuals. And although these ques- 

 tions have now lost much of their actuality through the gradual recognition by 

 naturalists that we have absolutely no criteria by which they can be settled, there 

 have arisen problems which promise quite as much interest and excitement for the 

 future. 



Volvox is much larger than the forms hitherto described. The colony is 

 spherical, and possesses a single layer of cells on its surface. In V. Globaior the 

 sphere may be over a millimetre in diameter, but more usually its diameter is only 

 some three-quarters of a millimetre. A particularly large specimen may possess as 

 many as 22,000 cells (10,000 is a more usual number). Many of the cells, however, 

 as in Pleodorina, are simply vegetative, and take no part in the reproduction of 

 the colony. Usually, in fact, only a very small minority of the cells are repro- 

 ductive. 



The two species of Volvox differ from each other in a great number of points, 

 but we have only space for a very brief description of some of the most interesting. 



The cells of Volvox Globator axe all united together by very stout processes. 

 Each cell, which is inclosed in a separate cell-wall, possesses all the ordinary features 

 of the Ghlamydomonas type. 



Daughter-colonies are developed from special cells, usually eight in number, 

 called parthenogonidia. They are always formed in the posterior part of the 

 mother-colony, early becoming larger than the ordinary vegetative cells. Each 

 divides repeatedly, and forms a hollow sphere of closely-packed cells, which, after 

 the last division, mostly acquire the characters of the adult vegetative cells, the 

 remainder gradually increasing in size to form the reproductive cells. The young 

 colonies then escape from the mother, apparently by pushing themselves against 

 and making rents in its posterior wall. Subsequently the cell-membranes swell a 

 good deal, separating the cell-bodies from one another, and the colony attains its 

 adult size. 



In other cases gametes may be formed in a young colony. About five cells 

 (androgonidia), strongly resembling the parthenogonidia, divide to form discs or 

 hollow spheres of a hundred or more spermatozoids. The spermatozoids resemble 

 in a general way those of Eudorina, but are peculiar in having the pair of flagella 

 inserted laterally at the base of the colourless beak and near the eye-spot. In the 



