166 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



consist of the two halves of the spore, and two new half cells, 

 which bear no resemblance to the spore. In Diatomads, 

 again, the conjugating frustules which have arisen from re- 

 peated cell division, are generally of very different sizes, as are 

 also the resultant frustules. In some species it seems matter 

 of necessity that the frustules which arise from repeated cell 

 division should become gradually smaller ; and in others, 

 whether necessary or no, it is a fact that they do decrease in 

 size, and the effect of the conjugation is to restore at length 

 the normal condition.* These differences should, therefore, 

 make us cautious in the proposition of new species. They are, 

 however, differences connected with the natiure of these or- 

 ganisms, and are not to be considered as real variations. Real 

 variations do, indeed, take place in the shape of the fronds 

 or frustules, the degree of their cohesion, the number of striee, 

 and other points which require to be taken carefully into con- 

 sideration. In Palmellece, the degree in which division 

 takes place, the number of the investing sheaths of mucus, 

 and the forms of the endochromes, vary so much, that species 

 ought not to be described on slight grounds, depending on 

 imperfect observations, which a glance at the microscope may 

 overthrow to-morrow. 



142. When we come to the articulated Algoe, amongst which 

 the distinctions of species are often slight, an increased degree 

 of caution is requisite. A very short acquaintance is sufficient 

 to show the immense difference of diameter which may exist in 

 threads of the same mass, and in the same threads the propor- 

 tions of length and breadth in the articulations are quite as 

 variable. Species, therefore, evidently of the most close affinity 

 cannot be safely separated from mere consideration of relative 

 proportion, without any other characters. Even the branching 

 of the threads is not sufficient, or the mode of branching. 

 Cladoj)hora glomerata assumes a multitude of forms which 

 it would be rash in the extreme to separate, and it may be 

 safely affirmed that of published species of Cladopliora and 

 Conferva, at least one half will ultimately be reduced. Where 



* See Carter, in Ann. of Nat. Hist., vol. xvii., p. 1, where many- 

 figures are given of conjugating Diatomacece. 



