274 



BOTANY: PRINCIPLES AND PROBLEMS 



small branch near by is cut off a cell which develops into an 

 antheridium. One of the sperms here developed enters the 

 oogonium and fertilizes the egg, producing a heavy-walled oospore. 

 4. Conjugales, the Pond Scums, Desmids, and Diatoms. — The 

 plant body of these algae is a single cell or an unbranched fila- 

 ment. The species are all confined to fresh water and are dis- 



FiG. 153. — Spirogyra. Two adjacent filaments, showing stages in sexual 

 reproduction. Cell a shows the normal, resting condition, with the spirally 

 placed, band-like chromatophore, in which numerous circular pyrenoids can be 

 seen. Cells b and c are sending out conjugating tubes to each other. The con- 

 tents of cells d and e have contracted somewhat and the contents of e is passing 

 over through the conjugating tube and uniting with d. At /is shown the mature 

 zygospore which has arisen from the union of two cells. X 150. 



tinguished from other Chlorophyceae by the absence of zoospores 

 or other means of asexual reproduction, the absence of motile 

 cells of any sort, the occurrence of large and conspicuous chloro- 

 plastids, and the characteristic manner in which sexual reproduc- 

 tion is brought about. 



