478 LIFE-HISTORIES 



the process of fission, by wliicli tliey multiply under ordinary con- 

 ditions. As shown in Fig. 308, the outer firm layer of the wall, 

 ruptures at the place of constriction, allowing the semicells to 

 separate, while the protoplasm of the isthmus or neck which joins 

 them elongates, being still covered by a thin, elastic, iimer layer of 

 cellulose. The nucleus and chromatophores divide, half going to 

 cither end. A partition is then formed across the middle of the 

 isthnms, and the new part on either side grows larger and rounder 

 until finalljr two separate and complete cells are formed each with 

 a new and an old half. Meanwhile chromatophores pass over into 

 this new semicell, and its outer wall becomes thickened and sculp- 

 tured like the other. In some desmids after fission the cells remain 

 attached forming a row comparable to the chain in Nostoc. A 

 striking peculiarity of the free forms like Cosmarivmi is their power 

 of locomotion. A\'hile not rapid, the movement is cjuite perceptible 

 under the microscope. It has been found to be accomplished by 

 protrusions of mucilaginous material, and its direction to be in- 

 fluenced by light. Resting spores are produced in the peculiar 

 manner shown in Fig. 300. Two cells Ijing side by side, each 

 rupture the outer wall across the middle and the halves separate 

 as for fission, but instead of forming a cross partition the protoplasm 

 of each plant flows out of the old cell-wall and toward the other 

 protoplast. When the two protoplasts come in contact they fuse 

 into a single spherical mass, the two nuclei merge into one, the 

 chromatophores disppear, and the whole contents becomes brown- 

 ish, while the outer cell-wall or erospore ' thickens and puts forth a 

 number of projections. In this condition the spore is comparatively 

 resistant of cold or dryness. On the return of favorable conditions 

 germinations take place as shown in Fig. 310. The contents sweU, 

 rupture the exospore, and emerge, surrounded bj^ the delicate inner 

 wall or endospore.- Soon a division of the nucleus takes place fol- 

 lowed by division of the protoplast into hah-es, which become 

 constricted, turn green and form cell-walls much like the uniting 

 pair from which they were derived. Freed from the endospore the 

 newly formed desmids swim about and multiply by fission, during 

 whicli process the chromatophores soon become distinct and the 

 cell-wall takes on its characteristic sculpturing. 



A spore formed as above described by the union of two similar 

 protoplasts is termed a zygospore,^ the uniting jirotoplasts arc known 

 as gametes,-^ and the process of their union is called conjugation.'' 



Closely related to the desmids are the "pond-scums," alga^ which 

 form tangled masses of delicate threads floating near the surface of 

 quiet fresh water. A very common genus is Spirogyra (Fig. 311) 



' Kx'o-spore < Gr. vxo, outside; sporos, spore. 



- En'du-spore < Gr. cndun, within. 



' Zy 'go-spore < Gr. zt/gon, yoke. 



'' Gam'ete < Gr. gniiifies, a siiouso. 



■' Con"ju-ga'tioii < L. cnn, together; ./(/(/arc, join or yoke. 



