QF THE DESMIDIACE^E AND SIMILAR MINUTE ALG.E. 53 



account with that from a spring, as its ' hard ' property is 

 sure to kill the plants. Care must be taken also not to ex- 

 pose the vessel to too much heat ; in fact, it had better be 

 set in the shady part of a window, where it is screened from 

 the direct rays of the sun. 



The student must not expect too much from this method 

 of multiplying the Desmids. It does well for a while ; 

 but there is no doubt that, in the course of time, the 

 plants degenerate, and cease to be fair representatives of 

 the species. 



[The figures (61 to 70) on Plate xm. will give the young 

 student an idea of the manner in which the Desmids 

 propagate themselves. They are examples of the common 

 constricted species, Cosmarium botrytis. 



In figs. 61 to 64 we see how the two portions of the 

 frustule gradually separate themselves, new cells forming 

 in the interior, which ultimately become perfect frustules. 

 This mode of propagation is known as ? cell division.' By 

 another method, called 'conjugation ' (of which more will 

 be said hereafter), two frustules, which chance to lie near 

 each other, form a temporary union, and mingle their 

 contents together, figs. 66 to 69. The mass takes an 

 irregular form at first, but gradually assumes the shape 

 given in fig. 70, viz. a globe covered with forked processes 

 (a, b). This globe, or sporangium, as it is named, gives 

 birth to new plants — Ed.] 



Where the observer is content to study the external 

 form only, the plan of simply drying his specimens on a slip 

 of glass is sufficient ; because, on re-moistening them, the 

 plants resume their former appearance. If, on the other 

 hand, he wishes to examine the peculiar arrangement of the 

 Chlorophyll), and the other contents of the cell, after the 

 object has been laid by, he must preserve them in some 

 fluid which will not alter their natural form and structure. 

 Much time also is saved by his having such preparations 

 ready to hand for comparison and observation. Moreover, 

 many of the smaller Confervas (such as Zygnema and 



