68 FECUNDATION ; NON-MOTILE ISOGAMETES. 



least, as a colony of individuals. Any cell of a filament, save those 

 mentioned, may function as a gamete. 



In sexual reproduction cells of two filaments lying close side by side 

 send out protuberances toward each other which meet end to end. In 

 the contiguous membranes a circular opening is made by the dissolu- 

 tion of the cellulose walls, through the agency of an enzyme, whereby 

 a continuous canal is formed between the cells (Fig. 20, A). It is 

 highly probable that the conjugating tubes are brought together by the 

 aid of a chemotactic, directive stimulus. Haberlandt ('90) claims, and 

 his view is shared by Klebs ('96), that the conjugating cells exert a 

 mutual chemical influence upon each other, namely, that a cell will 

 put out a conjugating tube only when influenced by another, probably 

 of a different sex, lying near it. In support of this view, Klebs found 

 that cells of individual filaments cultivated upon agar-gelatin, although 

 having been brought side by side by the folding of the filament, never 

 put out conjugating protuberances. A single male filament, on the 

 contrary, may conjugate with several female filaments whenever their 

 cells lie sufficiently near one another, but all those cells of the male 

 filament separated some distance from those of the female remain 

 sterile in spite of the tendency to conjugate. The limits of this mutual 

 action of the filaments (Haberlandt, '90) is equal to a distance of two 

 or three diameters of their cells. Slightly beyond this limit the cells 

 may put out short conjugating tubes, but these never reach each other, 

 the stimulus being presumably too weak. Haberlandt states further that 

 the conjugating tubes are not laid down simultaneously, but rather one 

 sends out a protuberance which calls forth the development of the cor- 

 responding tube from the other cell. If the protuberances do not lie 

 exactly opposite, they bend slightly in order to meet each other. A 

 further action of the stimulus is seen when a long male cell copulates 

 with two female cells. Two canals are formed connecting the male 

 with the two female cells, but, of course, only one of the latter receives 

 the gamete. In some species, especially Spirogyra injlata, according 

 to Klebs, the meeting of the conjugating protuberances is facilitated 

 by a curving or a knee-like bending of the cells, from whose convex 

 sides the protuberances arise. 



These phenomena are not presented in this connection for the purpose 

 of discussing any special phase of the physiology of the sexual process, 

 but merely to indicate a few features manifested by unisexual elements 

 which show a tolerably well-marked tendency toward bisexuality. 



When the conjugation canal, joining the gametes, is complete, the 

 turgor in each cell is diminished, so that each protoplast experiences a 

 self-plasmolysis. The contraction usually takes place first in the male 



