THE ORIGIN OF SEX IN PLANTS. 69 



or frequently 4 cilia at the pointed end. The illustrations show these 

 and other characters clearly. A portion of the protoplasm is differen- 

 tiated as a green body (chloroplast), which only partially fills the 

 rounded end of the zoospore, leaving the rest of the cell quite clear. 

 Sometimes the chloroplast contains a central body called the pyrenoid, 

 which is associated with the starch-forming activities of the chloroplast. 

 This structure must not be confused with the nucleus of the cell, the 

 latter being almost always invisible in the living zoospore. Finally, 

 one may always expect to find in the colorless pointed end, near the 

 cilia, a small bright red body called the pigment spot. The pigment 

 spot is generally believed to have a relation to the sensitiveness dis- 

 played by zoospores towards light, not in any sense, however, as an 

 organ of vision, as might be judged by the unfortunate term 'eye-spot' 

 that is sometimes applied to it. 



Such is the structure of the zoospores. Now let us consider their 

 habits. As we have said before, several are likely to be developed in 

 a single cell, but there is no rule as to number. Sometimes the entire 

 contents of a cell will slip out as a single zoospore, but more frequently 

 8, 16 or 32 will be formed, a variable number even in the same plant, 

 and in certain cases the parent cell will give rise to hundreds. The 

 zoospores escape from the mother cell or sporange usually through some 

 opening in the wall and immediately swim off. They may be developed 

 so numerously that the water is actually colored greenish and the field 

 of the microscope shows hundreds of these organisms moving rapidly 

 in various directions. Such appearances have given them the appro- 

 priate name of swarm-spores. The swarming of zoospores is best 

 shown under certain conditions of illumination. The zoospores are 

 very sensitive to light and usually arrange themselves with reference 

 to its source so that the long axes are parallel with the incoming rays. 

 If a vessel of water be placed so that the light rays come from one 

 direction, as from a window, the zoospores will move in parallel lines 

 towards or away from the source of the illumination. They will thus 

 collect in clouds in various parts of the vessel, the exact position being 

 somewhat modified by the currents of water that slowly circulate 

 through every brightly illuminated vessel. 



Generally speaking the swarm-spores that one is most likely to see 

 will be asexual. Their activities cease after a few hours or perhaps 

 minutes and they then attach themselves in some suitable position, 

 germinate and develop young plants called sporelings. Sporelings of 

 the alga, Ulothrix, are shown in Figure 2, d, they having developed from 

 a zoospore like c. 



But frequently and under conditions that have been in part deter- 

 mined swarm-spores will behave quite differently. They will swim at 

 first very actively, approaching one another and then darting awa}'. 



