Destt / idiacece 145 



attacks of small aquatic animals. After the loss of the filamentous 

 condition it became necessary for the solitary and unprotected 

 individuals to acquire some other means of defence, and presumably 

 the present morphological complexity is the result. It is a notable 

 fact that those species which occur on wet rocks and in other 

 localities in which Amoeba?, Oligochastes, Tardigrades, Crustacea, 

 etc., are either absent or very scanty, especially at high elevations, 

 usually possess a comparatively simple outline and are provided 

 with a more or less abundant mucus ; whereas those species oc- 

 curring in deep bog-pools, in the plankton, and the quiet margins 

 of deep lakes, in which localities such enemies abound, are generally 

 possessed of a more complicated, and in many cases of a formidable, 

 exterior. These characters acquired by the unicell are not only 

 protective against the depredations of aquatic animals, but are also 

 useful as anchors in the time of floods, and in their acquirement 

 the Law of Symmetry has exercised its full influence, with the 

 result that exquisite patterns have been produced which exhibit a 

 symmetry far ahead of that shown by any other living vegetable 

 organisms. 



There are several thousand known species of Desmids, about 

 one quarter of which are British, and almost all of them can be 

 readily identified by their external morphological features. Some 

 are cylindrical with rounded or attenuated apices ; many of them 

 are flattened and often disciform ; and others are of a radiating 

 character. To one who is not sufficiently versed in solid geometry 

 there are few greater surprises than the extraordinary aspects 

 presented by some of these plants. The majority of Desmids have 

 three principal axes of symmetry at right angles to one another, 

 and for this reason they require examining in three positions. 

 The most important aspect is the front view, in which the plant is 

 observed in that plane containing the two longest axes. The other 

 important aspects are the vertical view and the side (or lateral} 

 view (vide fig. 51 E and F). 



Desmids are subject to considerable variation, but only within 

 certain limits, and one of the most extraordinary facts relating to 

 these plants is the constancy of the markings embellishing the 

 exterior of the cell-wall. The following is a summary of the present 

 state of our knowledge concerning the variation of Desmids 1 : 



1 G. S. West, ' On Variation in the Desmidieas and its Bearing on their Classifi- 

 cation,' Journ. Linn. Soc. Bot. xxxiv, 1899, p. 376. 



W. A. 10 



