28 



fore follow, as Mr. Reade supposes, that these substances 

 must be regarded as constituent parts of the elementary or- 

 gans. M. Morren many years ago (1830) suggested the sup- 

 position that silica must be considered as a constituent sub- 

 stance of the cellular membrane, but this met with no recep- 

 tion ; for it is easily conceived that the presence or absence of 

 these alkalis and earths in the membrane depends entirely on 

 the contents of the inclosed fluids. 



The most important points respecting the occurrence of 

 crystals in plants may at present be considered as known. I 

 have given a very detailed account*, and M. Unger has also 

 published a memoirf on the same subject, accompanied with 

 a plate of beautiful drawings. M. Unger has confirmed the 

 statement, that the crystals are not formed in the intercel- 

 lular passages, but in the interior of the cells. I, on the other 

 hand, have observed several cases in which exceptions to 

 this rule occur. M. Unger states for instance, that the cry- 

 stalline glands, which are situated on the walls of the air- 

 cavities of Myriophyllum spicatum, are contained in distinct 

 projecting cells, and he has even figured these cellular walls ; 

 however, in all the cases which have come under my notice, 

 I have not been able, even with the most recent instru- 

 ments, to see the cellular walls which inclose the crystalline 

 glands ; this has also happened to Treviranus and Mirbel. 

 As however I can see with my instruments even the single 

 crystals of which those glands consist much more correctly 

 than M. Unger has represented them, I am inclined to think 

 that the cellular membrane figured as surrounding the cry- 

 stalline glands does not exist. 



In the work above cited I have enumerated several similar 

 cases where the deposition of crystalline masses takes place at 

 the exterior of the cells, and have drawn attention to the man- 

 ner in which the formation of crystals is to be understood. In 

 some cases, as in the Charce, the crystals form externally by 

 mere precipitation of the basal carbonate of lime, the carbonic 

 acid which held these in solution being absorbed by the plant. 

 If we place Conferva in lime water, small calcareous crystals 



* Neues System der Pflanzen-Physiologie, i. p. 212246. 

 f Ueber Krystallbildungen in den Pflanzenzellen. Annalen des Wiener 

 Museums, iii. p. 1. 



