XV111 INTRODUCTION. 



individual cells which enter into the structure of the higher 

 plants. 



The controversy respecting the nature of the Diatomaceae 

 may therefore be considered as determined in favour of their 

 vegetable character. 



If any doubt at present exist on the subject, it appears to 

 arise from the difficulty of reconciling the movements which 

 accompany the vital functions of the Diatornaceae with the pre- 

 valent conditions of plant-life, and of harmonizing the structure 

 of their siliceous epiderms with the ordinary forms under which 

 cellulose occurs throughout the vegetable kingdom. 



These points require elucidation. The isochronal movements 

 of the Diatomaceous frustule, already described (Introduction, 

 Vol. i. Sect. II.), have always presented a difficulty to their admis- 

 sion into the class of plants, with those who regard motion as a 

 special attribute of animal life : and the peculiarity of these 

 motions, even with observers who know that movement is of 

 frequent occurrence in the earlier conditions of vegetable deve- 

 lopment, has, perhaps unconsciously, been the occasion of doubt 

 and hesitation in forming positive conclusions respecting the 

 nature of such organisms. It ought, however, to be remem- 

 bered, that if these motions are singular in the vegetable king- 

 dom, they are unparalleled in the animal world, are wholly devoid 

 of those evidences of volition which are apparent throughout 

 the latter department of nature, and irreconcileable with the idea 

 of a living organism endowed, as are the lowest forms of the 

 animal being, with the power of voluntary movement. In 

 the vegetable kingdom alone we meet with actions present- 

 ing analogous phenomena ; the filaments of the Oscittatoriete, 

 while in a growing state, exhibiting motions, which, taking into 

 account the differences of structural arrangement, may be re- 

 garded as similar to those of the Diatornaceae. The excessive 

 rapidity of growth which characterizes the OscillatoriecB appears 

 a satisfactory explanation of the vibratory movements of their 

 filaments ; the same activity in their vegetative functions will go 

 far to explain the oscillatory motions of the Diatom, and to 

 harmonize the character of its frustule with the attributes of the 

 vegetable rather than the animal being. 



