I02 A TEXTBOOK OF THEORETICAL BOTANY 



around the pyrenoids in more or less spherical masses, thus giving the 

 impression of separate discoid chloroplasts, which may explain the view 

 expressed by some w^orkers that the chloroplast is not a single reticulate 

 structure but a number of small connected, discoid bodies. Fragmentation 

 of the chloroplast has been observed not infrequently. 



The wall of the coenocyte is composed of a series of thick, stratified 

 membranes, which consist of an inner and an outer lamella, with a superficial 

 pellicle which can be separated by treatment with acetic acid. Both lamellae 

 show stratification which usually runs obliquely to the long axis of the 

 coenocyte. There is little or no mucilage, which probably explains the great 

 abundance of epiphytes which normally occur on the plants. 



Branches usually arise at the upper end of a coenocyte and at first form 

 a wide angle with the parent branch, for the septum cutting oflF the branch is 

 developed nearly perpendicular to it. vSometimes two branches arise together, 

 one on either side of the main axis. 



Growth is by means of the apical segment, which, after elongating 

 sufficiently, forms a transverse septum cutting ofl^ the greater part of the 

 segment. At the beginning of such a division the protoplast is withdrawn 

 from the region where the new septum will develop, the space so left being 

 apparently filled with mucilaginous material. An annular bar of thickening 

 is laid down here, extending transversely all round the cell. This forms the 

 beginning of the cross wall which then gradually extends inwards towards the 

 centre. At the same time it extends into and fuses with the inner lamella of 

 the longitudinal wall. The formation of septa is in no way related to nuclear 

 division. 



The attaching organs consist of a number of septate, rhizoidal branches. 

 These may sometimes spread out as stolons which either give rise to new 

 upright filaments or their tips may grow out into a number of short cells, 

 which later fall apart and serve as a method of propagation. The short cells 

 formed on the rhizoids have thick walls and are abundantly supplied with food 

 reserves. After separation they may undergo further wall thickening and 

 function as over-wintering organs. Many species are, however, free-floating, 

 as can be seen in the floating tangles (" flannel weed ") which are so 

 characteristic of ponds. In the free-floating species smaller or larger parts 

 of the vegetative branches may separate, develop thick walls, and being well 

 supplied with food reserves may function, in some species, as the only means 

 of reproduction. 



Asexual Reproduction 



Every coenocyte of a branch is potentially capable of functioning as a 

 zoosporangium (Fig. 85), and it is by no means unusual to find all the cells 

 producing zoospores at the same time. Prior to the formation of zoospores 

 the chloroplast fragments and a gradual cleavage of the contents occurs in the 

 cell. Meanwhile the wall of the zoosporangium may begin to swell at one 

 side near its upper end. As this area continues to swell a rupture appears 

 at its centre. Through this the zoospores finally escape. 



