STILL AND MOTILE CONDITIONS OF PKOTOPHYTBS. 253 



substances are transformed into oil, which is stored up in the 

 seed for the nutrition of the embryo, and is applied, during ger- 

 mination, to the purposes which are at other times answered by 

 starch or chlorophyll. The growth of this little plant appears to 

 be favored by cold and damp ; its generation, on the other hand, 

 is promoted by heat and dryness; and it is obvious that the 

 spore-cell must be endowed with a greater power of resisting 

 this, than the vegetating plant has, since the species would other- 

 wise be destroyed by every drought. 



152. If the preceding sketch really comprehends the whole 

 life-history of the humble plant to which it relates, this history 

 is much more simple than that of other forms of vegetation, 

 which, without appearing to possess an essentially higher struc- 

 ture, present themselves under a much greater variety of forms 

 and conditions. One of the most remarkable of these varieties 

 is the motile condition, which seems to be common, in some 

 stage or other of their existence, to a very large proportion of 

 the lower forms of aquatic vegetation ; and which usually de- 

 pends upon the extension of the primordial utricle into one or 

 two thread-like filaments (Fig. 68, h-l), endowed with the power 

 of executing rhythmical contractions, whereby the cell is im- 

 pelled through the water. As an illustration of this peculiar 

 mode of activity, which was formerly supposed to betoken Ani- 

 mal life, a sketch will be given of the history of a plant, the 

 Protococcus pluvialis, which is not uncommon in collections of 

 rain-water,^ and which, in its motile condition, has been very 



' The Author had under his own observation, about eight years ago, an extraordinary 

 abundance of what he now feels satisfied must have been this plant, in a rain-water 

 cistern, which had been newly cleaned out. His notice was attracted to it, by seeing 

 the surface of the water covered with a green froth, whenever the sun shone upon it. 

 On examining a portion of this froth under the Microscope, he found that the water was 

 crowded with green cells in active motion; and although the only bodies at all re- 

 sembling them, of which he could find any description, were the so-called Animalcules, 

 constituting the genus Chlamydomonas of Prof Ehrenberg, and very little was known at 

 that time of the " motile" conditions o[ Plants of this description, yet of the vegetable 

 nature of these bodies he could not entertain the smallest doubt. They appeared in 

 freshly collected rain-water, and could not, therefore, be deriving their support from 

 organic matter; under the influence of light, they were obviously decomposing carbonic 

 acid and liberating oxygen, and this influence he found to be essential to the continu- 

 ance of their growth and developinent, which took place entirely upon the Vegetative 

 plan. Not many days after the Protophyte first appeared in the water, a few Wheel- 

 Animalcules presented themselves; these fed greedily upon it, and increased so rapidly 

 (the weather being very warm) that they soon became almost as crowded as the cells 

 of the Protococcus had been ; and it was probably due in part to their voracity, that the 

 plant soon became less abufidant, and before long disappeared altogether. Had the 

 Author been then aware of its assumption of the " still" condition, he might have found 

 it at the bottom of the cistern, after it had ceased to present itself at the surface. The 

 account of this Plant given above, is derived from that of Dr. Cohn, in the " Nova 

 Acta Acad. Nat. Curios." (Bonn, 1850), torn, xxii; of which an abstract by Mr. George 

 Busk is contained in the "Botanical and Physiological Memoirs," published by the Ray 

 Society for 1853. This excellent observer states that he kept his plants for observation 

 in little glass vessels, having the form of a truncated cone, about two inches deep, and 

 one inch and a quarter in diameter, with a fiat bottom polished on both sides, and 

 filled with water to the depth of from two to three lines. "It was only in vessels of 

 this kind," he says, "that he was able to follow the development of a number of various 



