212 VEGETATIVE REPRODUCTION [CH. 



individuality in the case of Phragmites commums, has brought 

 forward evidence which strongly suggests that the " major plant 

 unit," i.e. the total vegetative output which one fertilised egg 

 is capable of initiating, is to be regarded as a constant for each 

 species, its mass being the measure of specific vital energy. She 

 has shown that, in the case of the reed swamps of the Danube, 

 there are distinct indications of a definite life-cycle of vegetative 

 growth, terminating in senescence and death, whose arrival is 

 not fortuitous or due to external conditions, but is a necessity 

 inherent in the very nature of the species itself. Such a thesis 

 is obviously very difficult to substantiate, but the history of 

 Elodea, outlined above, certainly seems to the present writer 

 to lend itself more readily to some such interpretation, than to the 

 accepted explanation, which sees in the aggressive phase of this 

 introduced plant, merely the direct stimulating effect of change 

 of environment. Elodea has passed through a period of great 

 luxuriance, followed by a gradual diminution in vigour, occur- 

 ring more or less contemporaneously in all the localities which 

 have been colonised by its rapid vegetative multiplication. By 

 1883 its period of maximum abundance was apparently over. 

 In 1 909 an enquiry 1 was set on foot to determine the condition 

 of the species at that date, i.e. sixty-seven years from its first 

 recorded appearance in England. This enquiry resulted in reports 

 from many localities indicating that Elodea had sunk every- 

 where into the condition of a mere denizen, displaying no greater 

 luxuriance than the other water plants with which it was associa- 

 ted. Siddall, in this year, wrote that he had some difficulty 

 in finding a specimen of Elodea in a locality where in 1873 a ^ 

 other vegetation was choked with it. He also made the extremely 

 interesting statement that the circulation of the protoplasm was 

 very feeble in 1909 as compared with its condition in 1873 

 a statement which the present writer feels must be accepted with 

 some reserve, for it is a point on which a really critical com- 

 parison would be attended with obvious difficulties. 



The general history of Elodea seems at least to point towards 

 1 Walker, A. O. (1912). 



