70 CHLOROPHYCEAE 



growth and disappear under unfavourable conditions when growth 

 is slow. This is a feature of its metabolism that would seem to 

 require further investigation. The cells contain chloroplasts that are 

 discoid or band-shaped and devoid of pyrenoids. Usually both 

 prostrate and erect threads are present, though the latter are re- 

 duced in some species. Growth is apical, and the terminal cells 

 often bear a pectose cap or series of caps which are periodically shed 

 and replaced by new ones. The origin of the cap is not properly 

 understood but it is thought to be due to a secretion, whilst its 

 function may be either to reduce transpiration or else to act as a 

 means of protection: alternatively, it may simply be a means of 

 removing waste material. The cellulose walls are frequently 

 thickened by parallel or divergent stratifications, whilst each 

 septum between the cells may also have a single large pit which is 

 penetrated by a protoplasmic strand. The cells are uninucleate 

 when young and multinucleate when old, but the presence of the 

 pigment makes the nuclei extremely difficult to distinguish. 

 Vegetative reproduction is through fragmentation, whilst other 

 means of reproduction are to be found in three different types of 

 sporangia : 



(a) Sessile sporangia that never become detached. These consist 

 of enlarged cells which develop in almost any position and they 

 produce bifiagellate swarmers that may be isogametes. 



(b) Stalked terminal or lateral sporangia that are cut off from an 

 enlarged support cell which may give rise to several such bodies. 

 The apical portion swells out to form the sporangium and cuts off a 

 stalk cell underneath that frequently becomes bent. The dividing 

 septum possesses two ring-shaped cellulose thickenings which 

 may be connected with the detachment of the sporangium when it 

 is mature. The detached sporangium is blown away and germinates 

 under favourable conditions to bi- or quadrifiagellate swarmers. 



(c) Funnel-sh^Lped sporangia which are cut off at the apex of a 

 cylindrical cell, the outer wall splitting later at the septum, thus 

 liberating the sporangium, the subsequent fate of which is not 

 definitely known. The sessile and stalked sporangia may occur on the 

 same plant or else on separate plants. There has been no cytological 

 work to show whether there is any alternation of generations and 

 such an investigation would be highly desirable. In one species, 

 on the other hand, reproduction is wholly by means of aplanospores. 



