98 Biology in America 



an active swimmer. They also contain chlorophyl, enabling 

 it to manufacture its own food, so that physiologically it is 

 a plant, but in respect to the possession of cilia, and a red eye 

 spot which is sensitive to light, it resembles more nearly an 

 animal. Its reproduction is partly asexual and partly sexual. 

 In the former method, some cells multiply to form sec- 

 ondary colonies, which lie in the cavity of the mother colony 

 and finally break through its wall to form new colonies. In 

 the latter, certain large cells lacking cilia are differentiated 

 as eggs, while other cells divide to form a varying number 

 of motile sperms. Fertilization results in the formation of 

 a resting cell or "zygote," which after a period of inactivity 

 develops into a new colony. 



In definiteness of form, close association of cells and espe- 

 cially in the differentiation of sexual cells, Volvox stands as 

 a stepping stone between the unicellular types with their 

 typically asexual reproduction and the many-celled forms 

 which typically reproduce by fertilization. In yet another 

 respect does Volvox approach the higher types. Some species 

 are hermaphroditic, producing both eggs and sperms in the 

 same colony, while in others the two sexes are lodged in 

 separate individuals. There are many other forms, both 

 single-celled and colonial, which resemble animals in having 

 flagella and eye-spots, and plants in possessing chlorophyl. 

 Sometimes it is only the reproductive cells which have all 

 of these features, the ordinary cells being typical algae with 

 chlorophyl but neither flagella nor eye-spots. It is possible 

 that the * ' monads, ' ' to which reference has already been made, 

 have developed chlorophyl, giving rise to the plant kingdom, 

 on the one hand; and have assumed an ameboid form, pro- 

 ducing the animal kingdom on the other. This is suggested 

 by the occasional occurrence of flagellates which are either 

 ameboid at all times, or may assume an ameboid form at 

 certain times in their life cycle. 



Through the entire series of plants from the lowest to the 

 highest runs a curious phenomenon known as alternation of 

 generations, or the alternate succession of sexual and asexual 

 methods of reproduction. How many of us stop to think 

 when we pluck a violet or smell a rose that the flower was 

 not made to delight our eye or nose, but has developed as a 

 means for the perpetuation and increase of its kind? And 

 how does the flower perform its function? Hidden away at 

 its center, where but few of us ever see them, are the female 

 organs or ovaries, bearing at their summits little processes 

 known as styles, which end in small expansions, the stigmas. 

 Surrounding the ovaries are a ring of delicate filaments, the 

 stamens, each bearing at its tip a sack, the anther. In the 



