THE GREEN ALG^ 625 



of the chloroplast and are apparently of the nature of a 

 store of nitrogenous reserve-material ; under normal conditions, 

 however, they are remarkably persistent and serve as centres 

 for the formation of starch during assimilation. They are not 

 found in those forms which do not produce starch as the 

 result of photosynthesis. Except in the case of reproductive 

 elements, the algal cell is always enveloped by a well-defined 

 cell-wall, which often has a characteristic shape. A certain 

 number of the simpler unicellular forms of the green Algae 

 are motile during the greater portion of their life-history; the 

 movement is effected by the lashing to and fro of two or 

 more delicate protrusions (so-called cilia) of the protoplasm of 

 the cell or of each cell of the colonial forms, these cilia generally 

 arising close together (fig. 1, a). The cells of these motile 

 forms are generally also provided with a reddish eye-spot 

 (fig. 1, a, 5) which is situated in diverse positions, but always 

 in contact with the chloroplast, and seems to serve as an 

 organ for the perception of light-stimuli. Two peculiar 

 vacuoles (so-called contractile vacuoles, fig 1, a, v), which 

 are constant in position and are characterised by their rhythmic 

 alternate expansion and contraction, are also found side by 

 side just beneath the point of attachment of the cilia of the 

 motile forms. 



The reproductive processes of the green Algae show much 

 diversity, as will become evident from the subsequent con- 

 siderations, but at this point only a general outline will be given. 

 At the outset it is well to realise that all the different methods of 

 reproduction peculiar to each form depend on the realisation 

 of certain outside conditions (Klebs 40) and not on any inherent 

 tendency ; Klebs has shown in a considerable number of cases 

 that any particular method of reproduction can be called forth 

 at will by the introduction of certain appropriate external 

 conditions. The most important methods of reproduction are 

 the asexual and the sexual. In asexual reproduction the contents 

 of certain cells (which are known as zoosporangia, and may or 

 may not differ in shape and appearance from the ordinary 

 vegetative cells) contract away from the wall and are liberated 

 with or without previous division as motile cells, which are 

 known as zoospores (fig. 3a). The latter are generally more 

 or less pear-shaped, are provided with two or more cilia at the 

 pointed front end, and are naked (i.e. devoid of a cell-wall). 



40 



