CHAETOPHORALES 71 



Rowland (1929) has investigated the physiology of the commonest 

 species, T. aiirea, in some detail and he found that 



(a) drought increases the resistance to plasmolysis ; 



(b) if the threads are dried first and then heated together with 

 cotton-wool and gelatine, the results suggest that the threads hold 

 water in a manner similar to that of cotton-wool, but that the loss of 

 water on heating is comparable to that experienced by a colloid 

 or gel under the same circumstances (cf. fig. 49) ; 



(c) in damp, warm weather only small cells are formed because 

 cell division is relatively rapid ; 



(d) the threads can survive desiccation for at least six months; 



(e) plasmolysis could only be produced in some of the cells by a 

 25 % solution of sea salt. 



In many respects, e.g. the heterotrichous nature of the thallus, 

 the diiferent types of sporangia and the orange pigment, this alga 

 is strongly suggestive of the more primitive brown algae. This 

 feature, however, is discussed more fully in a later chapter (cf. 



P- 255)- 



*CoLEOCHAETACEAE : Coleochaete {coleo, sheath; chaete, hair). 

 Fig. 50- 

 Most of the species are fresh-water epiphytes attached to the 

 host by small outgrowths from the basal walls, but there is one 

 species that is endoph3rtic in Nitella, one of the Charales (p. 108). 

 Some of the species are truly heterotrichous whilst others only 

 possess the prostrate basal portion, which is either composed of 

 loosely branched threads or else is a compact disk. The growth of 

 the erect filaments is by means of the apical cell whilst the basal 

 cushion possesses a marginal meristem. Each cell contains one 

 chloroplast with one or two pyrenoids, and although a character- 

 istic sheathed bristle arises from each cell nevertheless in the old 

 plants these may be broken off. These bristles develop above a 

 pore in the cell wall through which the protoplast extrudes, whilst 

 at the same time a membrane is secreted over the protruding bare 

 protoplast. Asexual reproduction takes place in spring and early 

 summer by means of biflagellate zoospores which have no eye-spot 

 and are produced singly. After a motile phase the zoospore settles 

 down and divides either (a) horizontally, when the upper segment 

 develops into a hair and the lower forms the embryo disk, or 



