12 BOTANY. [chap. i. 



once expands and proceeds with its life work. As a 

 general rule, the simpler the structure of a living 

 organism, the greater its power of remaining alive under 

 extreme conditions, whereas organisms presenting a great 

 complexity of structure are limited to, comparatively 

 speaking, one set of conditions as regards temperature, 

 moisture, etc., and if removed from that sphere usually 

 perish. 



I. II. III. IV. V. 



Fig. I. Diagrammatic outline of Pleurococcus in various stages of 

 development. — I. Vegetative stage. II. First indication of reproduc- 

 tive stage. III. Same, further advanced. IV. A plant dividing into 

 four individuals. V. The four portions completely separated and 

 rounded off. (Highly magnified.) 



There are thousands of minute Algss that agree with 

 Pleurococcus in being unicellular and showing quite as 

 little division of labour or differentiation of structure, 

 the one cell having in turn to perform both vegetative 

 and reproductive functions. 



Prom this primitive condition of things we pass to 

 slightly higher types of structure where the individual 

 consists of a number of cells joined together, forming 

 either very slender simple or branched threads composed 

 of a single row of cells placed end to end, as in many 

 species of fresh-water Algae common in our ponds and 

 streams, and forming the bright green floating masses 

 so general during the summer; or the cells are arranged 

 to form a thin continuous sheet often of considerable 

 dimensions, as illustrated by many of the common red 



