96 PLANT LIFE. 



Alternation of generations is indicated in the Algse, but 

 does not become distinctly evolved, for although the major- 

 ity of species, as already stated, possess sexual and asexual 

 modes of reproduction, yet these do not regularly alternate ; 

 but there may be several successive generations of one method 

 without the intervention of the other. In many species there 

 is, throughout the summer or season of active growth, a suc- 

 cession of asexually produced generations; and in the autumn, 

 when the vegetative structure is languishing, sexual organs 

 are produced, the product of impregnation not unfrequently 

 passing the winter in a resting-stage. 



Coenobiese. 



The members of the present order are mostly microscopic, 

 and united in colonies to form a coenobium, the individuals 

 being usually imbedded in a mucilaginous matrix. Very 

 little is known respecting the life-history of the lowest types j 

 in the higher forms reproduction is effected asexually by 

 cell-division, or by zoospores ; sexually by the conjugation of 

 similar or dissimilar motile zoogametes, or by differentiated 

 antheridia and oogonia. 



The-following account of the life-history of typical forms 

 will illustrate the stage of development attained in the 

 group. 



Pandorina moricm is by no means rare during the summer 

 months in still, clear water, appearing under the form of 

 very minute green spheres involved in mucus. These 

 spheres, when examined under the microscope, are seen to 

 consist of sixteen cells or individuals containing bright 

 green chlorophyll, each furnished with two long, very 

 slender cilia, which protrude " through the mucilaginous 

 covering, and by their vibrations cause the entire coenobium 



