44 ELEMENTARY ORGANS Or PLANTS. 



forming in its interior. The green pulverulent matter which 

 appears on old walls, and on the hark of trees, consists of an 

 unformed mass of free globular cells, which grow and repro- 

 duce in this simple manner. Here we have the starting- 

 point of vegetation, the beginning of the formation of those 

 vegetable elements which, in their future development, shall 

 clothe the earth's surface with the richest forms of life and 

 beauty. In other species of plant-cells, the mode of repro- 

 duction is somewhat different. In Chroococcus rufescens, 2, 

 the plant-cell takes an oval form; a partition then appears 

 across its cavity, as represented at 3, dividing it into two cells. 

 These two cells are again subdivided by the formation of 

 another septum at right angles to the first partition, as is seen 

 at 5. The four cells thus formed enlarge and ultimately sepa- 

 rate, constituting four new individuals, which propagate in like 

 manner. In Osoillaria, the plant-cell becomes elongated, or it 

 may become elongated and branched, as is the case with the 

 species Vaucheria, which forms one kind of those delicate and 

 flossy green threads abounding in fresh water, and which are 

 popularly known in some places as brook-silk. Fig. 18 is a 

 magnified view of Vaucheria clavata, which consists of a sin- 

 gle cell of unbroken caliber, furnished with branches. In one 

 of these branches, at a, a spore is forming, b represents the 

 end of the branch more magnified, with the spore escaped 

 from its burst apex. In this instance, the ramifications of the 

 cell foreshadow, as it were, the elongation and ramification of 

 cells in the stem and branches of more highly organized 

 plants. 



