216 VEGETATIVE PROPAGATION 



unfavourable to vegetative growth (e.g. the cold months of the 

 winter), the cells of this Alga often become laden with food- 

 reserves and develop exceptionally thick walls. 



All the higher Algae resemble Ulothrix and Cladophora 

 in the restriction of motility to the reproductive phase, which, 

 however, exhibits a varying degree of specialisation in the different 

 forms. Vegetative propagation by fragmentation (i.e. cell- 

 separation without preparatory division), as among aquatic 

 Flowering Plants, is very common. In Spirogyra, for instance, 

 the threads not uncommonly break up into their constituent 

 cells, each of which can divide to form a new filament. In 

 the common soil Alga Hormidium this is the customary method 

 of propagation during a great part of the year (Fig. 116, C). 

 The thick-walled threads of Cladophora above mentioned often 

 fragment in a similar manner, when renewed growth takes 

 place. Formation of new individuals by cell-division is the 

 commonest form of reproduction in Desmids, Diatoms, and the 

 unicellular Blue-green Algae, whilst the filamentous members 

 of the last-named group propagate abundantly by mere frag- 

 mentation. In such massive forms as Fucus, the same end is 

 attained by the detachment of small adventitious branches 

 of the thallus, which are often formed in bunches at points of 

 injury, and are especially characteristic of the unattached species 

 of this genus which occur on salt-marshes. 



The majority of the Green Algae, with the exception of the 

 Conjugatae, reproduce asexually by means of zoospores, but 

 these often possess a more elaborate structure than those of 

 Ulothrix. For example, in (Edogonium, where they are pro- 

 duced singly from the ordinary cells, they are much larger and 

 bear a ring of cilia a little way behind the colourless front end 

 (Fig. 117, e). If filaments of this Alga are brought indoors, 

 zoospores are usually formed within a few hours, and, with a 

 little patience, their development and liberation can be observed. 



In the first place the protoplast contracts slightly away from 

 the wall, and soon after this a colourless area, marking the future 

 front end of the zoospore, arises on one side of the cell (Fig. 117, b). 

 Around the edge of this area the numerous short cilia sprout 

 out, appearing as so many fine lines. Thereupon the wall 

 breaks across, near one end of the cell, and the shorter piece 



