DIV. I 



MORPHOLOGY 



79 



Such an apical cell divides by transverse walls parallel to one another, which 

 cut off disc-shaped segments from its lower end. These divide further in a regular 

 way, first by longitudinal and then by transverse walls into a number of cells, 

 which are at first meristematic. The lateral branches, mostly developed as 

 shoots of limited growth, develop from lateral cells in acropetal succession, and 

 give the characteristic form to the plant (Fig. 89). Flat ribbon-shaped thalli 

 may have a similar but correspondingly flattened apical cell, as seen in the Brown 

 Seaweed, Dictyota dichotoma ( 5a ) (Fig. 90). Flat segments are cut off from this by 

 walls convex backwards, and are then divided by longitudinal walls. Sometimes 

 the apical cell is divided by a longitudinal wall into two cells of equal size lying 



' 



FIG. 88. Delesseria sanguinea. (i nat. size. 

 After SCHENCK.) 



FIG. 89. Cladostephus verticillatus. (x 30. 

 After PRINGSHEIM.) 



side by side (Fig. 90 B, a, a), each of which forms one of the branches of the 

 dichotomy. 



The permanent cells even of highly -differentiated thalli almost 

 always have the characters of parenchyma. There may be a distinction 

 of peripheral assimilatory parenchyma with abundant chlorophyll, 

 storage parenchyma, colourless and with abundant reserve materials, 

 and conducting parenchyma composed of elongated cells. 



Since the multicellular Algae living in water do not require protection against 

 drying up, and when exposed to the air at ebb-tide are protected by a covering of 

 mucilage, a typical epidermal layer is wanting. The Algae show, however, an 

 outer lamella of the cell walls of their superficial cells, which stains brown with 



