28 



OUTLINES OF EVOLUTIONARY BIOLOGY 



in the same proportions as in water (H 2 0). Thus the chemical 

 formula for cellulose is (C6Hi 5 ) n , while that for glucose or grape 

 sugar, another carbohydrate, is CeHiaOe- The presence of very 

 definite cell-walls, composed of cellulose and formed as a secretion 

 by the living protoplasm, is very characteristic of vegetable as 

 contrasted with animal organisms. 



The protoplasm which lies inside the cell-wall is, as we have 

 already said, either red, green or parti-coloured. The green 



FIG. 5. Structure and Life-history of Hcematococcua plttvialis. 



A. Resting stage with thick cell-wall. 



-B. Division into four motile zoospores within the old cell-wall. 



C. Free-swimming zoospore. 



D. Division of the resting cell into 32 microzooids or gametes. 



E. Free-swimming gamete. 



F G. Conjugation of two gametes. 

 H. Zygote with four flagella, formed by conjugation. 

 J. Zygote with flagella withdrawn. 



K. Resting cell formed from the zygote by secretion of a thick cell-wall. 



c.w. cell-wall ; fl. flagellum ; mi. nucleus ; py. pyreiioid ; vac. vacuole. 



(Figs. D K adapted from Peebles.) 



colour is due to the presence of that extremely characteristic 

 vegetable pigment known as chlorophyll, a remarkable pro- 

 duct of the activity of the living protoplasm with which we are 

 all familiar in the case of ordinary green plants. The red 

 pigment, known sis haematochrome, is but a slight chemical 

 modification of the green chlorophyll, and the one may readily 

 be converted into the other. If some nitrogenous substance, 

 such as a small quantity of manure water, be placed in a 

 vessel containing red Hsematococcus, the latter will in a short 

 time assume a bright green colour, whence we may conclude that 

 the red colouration is probably an effect of nitrogen starvation. 



