138 BOTANY. 



ditches is somewhat higher in structure than Pandorina, which it 

 resembles in many respects. Volvox is a 

 colony of very many little cells, each of 

 which projects its two cilia outward, giv- 

 ing the ball a hairy appearance. By the 

 lashing of the cilia the ball rolls about in 

 the water. At a certain stage some of the 

 cells enlarge and slip into the interior of 

 the colony, becoming free oOspheres, each 

 containing one germ-cell. At the same 

 time other cells break up their protoplasm 



into motile antherozoids, which escape into 

 Fig. 68.— a Volvox col- ,, '^ c ^x. ^ a ^ i ii, 



ony, magnified about 45 the same cavity of the colony. At length 



times, showing young cole- the antherozoids unite with the oospheres, 

 when as a result the latter secrete thick 

 walls, and thus become resting spores. Upon germination each rest- 

 ing spore divides its protoplasm into several hundred small cells, 

 which then arrange themselves into a new colony. The asexual re- 

 production takes place by certain cells breaking into great numbers 

 of little cells, which then unite themselves directly into a new colony 

 in the interior of the parent colony (Fig. 68). 



Practical Studies. — (a) In midsummer collect a few quarts of the 

 surface water of weedy ponds; together with the pond-scums grow- 

 ing therein ; put it into a shallow dish, and after an hour or so look 

 carefully (with the naked eye) for Volvox. It will be seen as a 

 minute green ball (from .5 to 1 millimetre in diameter) rolling slowly 

 through the water. Now carefully transfer it to a slide along with 

 enough pond-scum to prevent crushing. Under a low power even 

 many of the details of structure may be made out, and one or more 

 young colonies in the interior may almost invariably be seen. 



(6) In similar situations Pandorina may be obtained for study. 



Systematic Literature. — Wolle, Fresh-water Algae of the United 

 States, 156-163. 



Order 4. C01TJTJGAT.a;. Thb Pond-scums. 



242. Here the sexual cells which unite are fixed ; that 

 is, they are not locomotive. The sexual act always takes 

 place in the mature plant. No zoospores are produced. 

 This order includes many plants of great beauty and scien- 

 tific interest. Of the five families here noticed the first 

 three are composed of chlorophyll-bearing plants, while in 

 the fourth and fifth they are destitute of chlorophyll. 



