258 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



156. What are the precise conditions which determine the 

 transition from one state to the other, cannot yet be precisely 

 stated ; but the influence of certain agencies can be predicted 

 with tolerable certainty. Thus it is only necessary to pour the 

 water containing these organisms, from a smaller and deeper 

 into a larger and shallower vessel, at once to determine segmen- 

 tation in numerous cells, a phenomenon which is observable 

 also in many other Protophytes. The "motile" cells seem to be 

 favorably affected by light, for they collect themselves at the sur- 

 face of the water and at the edges of the vessel ; but when they 

 are about to undergo segmentation, or to pass into the " still" 

 condition, they sink to the bottom of the vessel, or retreat to that 

 part of it in which they are least subjected to light. "When kept 

 in the dark, the "motile" cells undergo a great diminution of 

 their chlorophyll, which becomes very pale, and is diffused, in- 

 stead of forming definite granules ; they continue their move- 

 ment, however, uninterruptedly, without either sinking to the 

 bottom, or passing into the still form, or undergoing segmenta- 

 tion. A moderate warmth, particularly that of the vernal sun, 

 is favorable to the development of the u motile" cells ; but a tem- 

 perature of excessive elevation prevents it. Eapid evaporation 

 of the water in which the " motile" forms of Protococcm may be 

 contained, kills them at once ; but a more gradual loss, such as 

 takes place in deep glasses, causes them merely to pass into the 

 " still" form ; and in this condition, especially when they have 

 assumed a red hue, they may be completely dried up, and may 

 remain in a state of dormant vitality for many years. It is in 

 this state that they are wafted about in atmospheric currents, and 

 that, being brought down by the rain into pools, cisterns, &c., they 

 may present themselves where none had been previously known 

 to exist; and there, under favorable circumstances, they may 

 undergo a very rapid multiplication, and may maintain them- 

 selves until the water is dried up, or some other change occurs 

 which is incompatible with the continuance of their vital activity. 

 They then very commonly become red throughout, the red color- 

 ing substance extending itself from the centre towards the cir- 

 cumference, and assuming an appearance like that of oil-drops ; 

 and these red cells, acquiring thick cell-walls and a mucous en- 

 velope, float in flocculent aggregations on the surface of the 

 water. This state seems to correspond with the "winter spores" 

 of other Protophytes ; and it may continue until warmth, air, 

 and moisture, cause the development of the red cells into the 

 ordinary " still" cells, green matter being gradually produced, 

 until the red substance forms only the central part of the en do- 

 chrome. After this occurs the cycle of changes which has been 

 already described ; and the Plant may pass through a long series 

 of these, before it returns to the state of the red thick- walled cell, 

 in which it may again remain dormant for an unlimited period. 

 Even this cycle, however, cannot be regarded as completing the 



