272 



Introduction to Botany. 



leaves of higher plants. The student may be able to per- 

 ceive some advantage in the spiral course of the elongated 

 chloroplasts with reference to the absorption of light. 



All cells of the filamentous individual 

 perform the same nutritive functions ; 

 any one of them may take part in the 

 formation of spores ; and all bear prac- 

 tically the same relation to the outer 

 world. The exterior wall of the filament, 

 and the partition walls between the cells, 

 are of cellulose, and permit liquids and 

 gases to pass readily through them. 

 Since they possess no waterproof cover- 

 ing, the filaments quickly dry up when 



taken out of the 



V water. Being 



buoyed up by 

 the water and 

 freely floating in 

 it, very little 

 stress is ever 

 exerted on the 

 filaments, and 

 special strength- 

 ening devices 

 are unnecessary. 

 165. Repro- 

 duction of Spi- 

 rogyra. — Spirogyra shows a degree of sexuality in the 

 method of its reproduction. Two filaments which happen 

 to be lying in close proximity put forth outgrowths from 

 one or more of their cells which finally meet, and the walls 

 separating them become absorbed. The entire protoplast 



Fig. 137. 



V, a single cell of Spirogyra, showing the spiral chloro- 

 plasts containing numerous rounded pyrenoids. The 

 nucleus is suspended at the center of the cell, w and 

 X, two conjugating filaments ; at i, an early stage in the 

 formation of a connecting tube between the two cells; 

 at 2, the tube has formed and the protoplast from a cell 

 of filament w is passing into the corresponding cell of 

 filament x and is fusing with its protoplast ; at 3, a later 

 stage ; at 4, a spore has formed from the fused proto- 

 plasts. After Sachs. 



