Cell-wall 127 



(consult figs. 149, 152 and 165). Except for the attenuation of the branches 

 into hairs in some species, there is practically no differentiation among the 

 vegetative cells, but there may be a marked distinction between the vege- 

 tative and reproductive cells. 



The CELL-WALL varies much in its structure in the different groups 

 of the Chlorophycese. It is in most instances composed of cellulose, but 

 sometimes it consists largely of pectose. In all cases it is a secretion of 

 the protoplasm, and arises on the outer surface of the protoplast as the 

 result of complex katabolic processes. Many of the thicker walls are 

 lamellose, the lamination being most easily seen after treatment by strong 

 acids or other hydrating reagents which cause the wall to swell. These 

 lamella? represent successive layers of growth in thickness, and in the 

 case of thick walls they are often very numerous. They are not always 

 parallel to the outer surface of the cell-wall, but may be upwardly and 

 outwardly divergent. The pectose compounds are sometimes sharply de- 

 marcated from the cellulose parts of the wall, and at other times the 

 two constituents are to some extent in alternating layers. The cellulose 

 parts give a distinct violet colouration with chlor-zinc-iodine, whereas the 

 pectose constituents do not. In many of the Green Algae the pectose 

 constituents of the wall are in the form of gelatinous layers on the outside 

 of the much more compact layers of cellulose. There is a copious secretion 

 of jelly by large numbers of the Green Algae, but the chemical composition of 

 these gelatinous masses cannot yet be said to have been satisfactorily deter- 

 mined. Virieux ('10) considers that the mucilage in most of the Chlorophyceas 

 is of a pectic composition, although in two cases he found in addition mucilage 

 of the nature of callose. It is highly probable in many cases that much of 

 the mucus arises by the conversion of the outer layers of cellulose into 

 mucilaginous substances of various degrees of solubility in water, and incre- 

 ments are constantly added by the gelatinization of successive layers. The 

 refractive index of the jelly differs very little from that of water, and very 

 often the extent of the gelatinous investment can only be detected by 

 staining, or by immersion of the Algae in Indian ink in the living state. 

 The mucus stains with various aniline dyes such as fuchsin, safranin, 

 methylene-blue, and gentian-violet. It is also coloured more or less deeply 

 by ruthenium red. 



Sometimes the gelatinization occurs only on one side of the wall, with 

 the result that the cells appear to be stalked, as in Hormotila (fig. 115 C). 



It is in the majority of the Protococcales, and in the Conjugatae, that the 

 cells, colonies, or filaments are so conspicuously enveloped in a gelatinous 

 covering. In the former group the jelly is very largely the result of the 

 gelatinization of the outer layers of the wall, whereas in most of the Con- 

 jugatse the mucilage is secreted directly by the protoplast and passes through 



