3 66 BOTANY OF THE LIVING PLANT 



the water (B). The cell which produces them may be called a zoo- 

 sporangium. According to the number of the divisions the zoospores 

 may differ in size. The large macrozoospores have each four cilia 

 attached to the narrower end of its pear-shaped body (C) ; the smaller 

 microzoospoies have four or two cilia. After a period of movement, 

 they settle, form a cell-wall, and affix themselves to some solid sub- 

 stratum : growing out transversely to their former axis and dividing, 

 each may form a new filament. The gametes are also produced in a 

 similar way, from a cell which may be called a gametangium : but their 

 divisions are more numerous, their size smaller, and they bear only 

 two cilia (E). The gametes, which are all alike in size and form, 

 escape from the cells : if those from different filaments meet they 

 coalesce in pairs, the result being a four-ciliate zygote, which soon loses 

 its cilia, settles and forms a cell-wall. After a period of rest it 

 germinates, the contents dividing, and escaping as zoospores, which 

 grow into new filaments (/, K) ; the first division is a reduction- 

 division. 



Ulothrix takes a low place both as regards structure and propagative 

 method. The differentiation of its sexual cells is imperfect. Not 

 only is there no distinction of sex in the form of gametes, but 

 occasionally the gametes may themselves germinate without fusion. 

 They are strikingly similar in form and origin to the zoospores. The 

 facts are in accord with the theory that fusion of gametes (syngamy) 

 is a means of strengthening otherwise weak cells, which were originally 

 organs of vegetative propagation. 



Oedogonium and Bulbochaete are also filamentous Algae, but with more 

 elaborate structure of their cells. Various species are very commonly 

 found attached to stones or submerged parts of plants in quiet fresh 

 water. Their cells are uninucleate, and contain a single reticulate 

 chromatophore. They may be propagated by motile cells, or zoo- 

 spores, which are formed from the whole content of a cell. Each 

 escapes through a transverse slit in the wall into water, having an 

 oval form, with a fringe of cilia round the colourless anterior end 

 (Fig. 271, A, B). After a period of movement the zoospore settles, forms 

 a cell-wall, and grows directly into a new individual. The plants are 

 readily distributed by this means. The sexual organs of Oedogonium 

 are antheridia and oogonia, which differ in size. The oogonium is a 

 large barrel-shaped cell, containing a single egg. It opens at maturity 

 by a transverse slit, as in the liberation of the zoospores ; but the 

 ovum remains in situ, and is motionless (Fig. 271, C, D). At the same 

 time cells, of the same of of a separate filament, undergo repeated 



