480 



LIFE-HISTORIES 



in which the plants are unbranched filaments consisting; of a single 

 row of comparatively large cylindrical cells containing spiral, 

 ribbon-like chromatophores, ratlusr j)rominent pjTenoids, and co- 

 pious cell-sap. Elongation of the thread results from fission of 

 the cells much as in Nostoc; but with Spirogyra the more intimate 

 union of adjacent cells gives rise to a multicellular individual rather 

 than to a colony of unicellular plants. Conjugation takes place 

 between cells of separate plants growing near together. Outgrowths 

 from two opi^osite cells meet, and by absorption of the walls at the 

 point of contact form a tube connecting the cavities. Tlirough this 



Fig. 312. — Pond-scum. Germination of zygospore. I, resting zygospore; 

 /, yellowish-brown layer of the cell-wall; e, outer layer of wall. II, the 

 protoplast emerging from the old wall, covered by a thin wall (g) of 

 its own. Ill, young plant beginning to form a thread-like row of cells 

 by elongating and forming cross-partitions (u\ tv') ; its small end (d) 

 still within the old spore-wall, enclosed by the old cell-wall of the 

 original plant whose conjugating-tube (c) is still visible. Much magni- 

 fied. (Pringsheim.) 



tube the gamete from one cell passes into the other cell to form a 

 zygospore. Germination takes place in the manner indicated by 

 Fig. 312. 



What is here especially noteworthy is that in the conjugation of 

 Spirogyra we have a simplest form of sexual reproduction. Whereas 

 in Cosmarium the gametes are just alike, in Spirog.vra the one which 

 passes over might i)e called male, and the other, female, although 

 before conjugation no difference between them is perceptible. 



Somew^hat more highly developed in both the vegetative 

 and the reproductive system are the green algae of the genus 

 Ulothrix (Fig. 313) which consist of cylindrical cells forming 

 an unbranched filament fastened by one end to a rock or other 

 firm support. These filaments grow so crowded as to form 



