J 26 INTRODUCTION TO CRYPTOGAMIC liOTANY. 



myself, Mr. Broome and others, leaves no doubt that they 

 belong to the same order of beings as JDesmidiacece and Gonjit- 

 gatce. The process of conjugation as described by Mr. Thwaites 

 in Eunoiia turgida, takes place in the following way. The 

 two frustules being brought near to each other by their concave 

 surfaces, two little swellings arise in each, meeting two similar 

 ones in the opposite frustule. These soon unite and elongate ; 

 the dissepiment is absorbed, and the endochromes of the 

 two frustules mix. A spore is then formed in each of the two 

 connecting tubes, which increases in size till it resembles in 

 every respect the parent, except in its much larger size. In 

 the same grouj^, therefore, frustules of very different sizes may 

 occur, especially as the new frustule will divide several times 

 before it acquires the normal dimensions of the species. In 

 some species, as in those which form threads, the new indivi- 

 duals do not resemble the old, but put on very different forms, 

 as in the Desviidiacece, and have in consequence been referred 

 by authors to different genera. A large quantity of mucus is 

 often generated during the process, in consequence of which, 

 as in our figiire, the old and new frustules are bound together 

 in a common group. This jellj^ is of consequence in deter- 

 mining the difference between old and new frustules in those 

 genera in which they do not differ materially from each other. 

 In Fragilaria, only a single spore is formed. In every 

 case, the two parent frustules are split in half by the swelling 

 of the spore. In some cases the new spores acquire a stera, 

 even where the frustules are not normally stipitate. At 

 present, as far as I know, active granules have not been 

 discovered in the cells ; and it appears that true spores are 

 far less common here than among the group just described. 

 The species are far more widely diffused, and extend beyond 

 the limits of all other vegetation, nor is there probably a 

 j)ortion of the world where there is sufficient water to allow of 

 their production, from which they are entirely excluded. They 

 flourish both in standing and running water, but they are 

 found, also, on the bare surface of the ground, or on objects, as 

 rabbits- dung, lying upon it ; and in South America they occur 

 amongst lichens, upon the trunks of trees. While, on the one 



