262 



Siphonodadiales 



and an outermost layer which can be readily detected by treatment with acetic 

 acid (Brand, '01 ; consult fig. 167 C F). In some species, even in the living 

 cells, the wall exhibits a cross-striation, and Brand's investigations ('06) show 

 that this is caused by the presence of fibres and fibrils, which although not 

 interwoven, pass near and over each other. 



The branches arise as outgrowths of the upper part of the lateral wall of 

 the parent-segment, and by growth on the part of 

 the parent-segment are upwardly displaced so that 

 eventually a dichotomous or polytomous appearance 

 may result (Brand, '99 ; '01). 



Each segment of the thallus contains many 

 nuclei, although in some cases the number is much 

 reduced. The chloroplast is normally a parietal 

 reticulum (fig. 168), but under abnormal con- 

 ditions and in segments which are degenerating 

 all transitions occur between a true network and 

 numerous isolated plates (Brand, '01). 



In the division of the thallus into segments the 

 transverse walls are formed by the ingrowth of a 

 ring-like septum which originates from the inner 

 layer of the cell-wall only (fig. 167 C F), After 

 the completion of the transverse wall a definite 

 'joint ' is formed by the formation of a 'joint-space.' 

 The joints ofCladophora have been carefully studied 

 by Brand ('01 ; '08), who finds that the 'joint- 

 space ' between the inner and outer layers of the 

 cell-wall is filled up by loosened lamellae of the 

 inner layer (fig. 167 F). This development of 

 definite joints between the segments of the thallus 

 adds much to the elasticity and flexibility of the 

 filaments. 



Vegetative propagation occurs by disjointed parts of the thallus (termed 

 by Wittrock ' prolific cells ' in Pithophora) and by large ccenocysts (which are 

 really hypnocysts). 



Asexual reproduction takes place by biciliated zoogonidia formed in 

 large numbers in the segments of the thallus. They may arise in almost 

 any segment of the thallus and they escape through an opening which arises 

 by a complete dissolution of the cell-wall at some point near the apex of the 

 segment. According to some observers the cilia are not always of equal 

 length. 



Gamogenesis occurs by the fusion of isoplanogametes, and the zygote 

 germinates at once. 





Fig. 168. Upper part of seg- 

 ment of Cladophoraglomerata 

 (L.) Kiitz. stained to show 

 nuclei (ri) and pyrenoids(p?/). 

 The reticulate character of 

 the chloroplast is also well 

 shown, x 610. 



