LEIPNER, ON SILICA IN PLANTS. 137 



I have already referred to the small and variable quantity 

 of silica present in the Rubiacese, and propose to take advan- 

 tage of this pecidiarity in further investigations, by which I 

 hope to arrive at more positive conclusions upon the vexed 

 question of the nature of siliceous deposits ; whether they 

 obey the ordinary laws of inorganic bodies, and are simply 

 deposited from the secretions in the manner of raphides, as 

 Schleiden appears to think, or whether they are deposited 

 pari passu with organic atoms as an integral part of the 

 cell-growth, and in conformity with organic developme^it. 

 In confirmation of the latter supposition, I may here slightly 

 advert to a peculiarity which I have noticed in treating 

 siliceous membranes with nitric acid. If the action is main- 

 tained only so long as is required to destroy the adhering 

 organic substances, a siliceous membrane perfectly homoge- 

 neous in its structiQ'e is presented ; but if this membrane be 

 further treated with nitric acid, it loses its homogeneous 

 appearance, and seems to be studded all over with small 

 granules, and the more so the longer it is boiled in the 

 acid. Tliis can be better seen by changing the focus, as 

 some of the granules are more deeply imbedded in tlie mem- 

 brane than others. 



In concluding this communication I may state that I 

 have found silica in every one of the four genera of which 

 the British Rubiacese are composed, viz., Rubia, Galium, 

 Sherardia, and Asperida ; in fact, although I have examined 

 all indigenous plants of this order, with the exception of 

 Galium pusillum (L.), G. erectum (Huds.), and G. Vaillantii 

 (D. C), I have failed to distinguish the silica-deposit only 

 in Galium cruciatum (L.) In all of the others, I have 

 found siliceous epidermic tissue in the stem as well as in the 

 leaves, which depolarizes light if it is present in sufficient 

 thickness ; this is especially the case in the prickles, which 

 all these plants have at the margin of the leaves and the 

 angles of the stem. I have also detected it in the cotyledon- 

 leaves of Galium mollugo (L.), the only species in which I 

 have examined this organ. 



That the Rubiaceae are not the only exogenous order in 

 which silica is to be sought will appear from the fact, that 

 I have also found it in Achillea ptarmica (L.), one of the 

 Compositse. This plant, which bears the popular name of 

 " sneeze-wort yarrow," possesses a particularly large amount 

 of silica in the prickles forming the sharp double seiTatures 

 of the leaves, which is doubtless the cause of its errhine pro- 

 perties when powdered and used as snuff. 



