402 ELEMENTARY BIOLOGY. [CHAP. j 



This process, to which the name of monoecious conjugation 

 may be applied, occurs in the same species which are usually 

 reproduced by the normal ladder-like or dioecious con- , 

 jugation. fi 



Some allies of Spirogyra show no demonstrable difference 

 of sex between the conjugating cells, for the zygospore is 

 formed in the connecting passage, each cell thus appearing 

 to take an exactly equal part in the process. 



The mode of reproduction in Spirogyra and its allies, simple 

 as it is, may serve as the type of all sexual reproduction. 

 Among the innumerable modifications under which the 

 sexual process takes place, the one point which is found to 

 be constant is the fusion of the nuclei of two distinct cells. 



A few words on the Physiology of Spirogyra may be added. 

 Its nutrition takes place in all important respects in the 

 same way as that of Protococcus. The carbonaceous food 

 of the plant is obtained by decomposition of the carbon 

 dioxide dissolved in the water, and this of course only goes 

 on under the influence of light. Starch is the first easily 

 demonstrable product of this process, though it is certainly 

 not the first product actually formed. It is easy to show 

 that the formation of the starch-grains is dependent on the 

 action of light. If the plant be kept for some hours in the 

 dark all the starch will be found to have disappeared. A 

 very short exposure to daylight is sufficient to induce its 

 renewed formation. Under the action of direct sunlight a 

 demonstrable amount of starch may be developed within so 

 short a time as five minutes. It will be remembered that 

 the starch-grains are formed around the pyrenoids. As re- 

 gards the part played by the latter in the process we have 

 no definite knowledge at present, but similar proteid crys- 

 talloids are found elsewhere in connection with starch- 

 forming corpuscles. 



