412 ON CONCRETIONS, SPICULES, [ch. 



So far as the problem is a purely chemical one, we must deal 

 with it very briefly indeed; and all the more because special 

 investigations regarding it have as yet been few, and even the 

 main facts of the case are very imperfectly known. This at least 

 is evident, that the whole series of phenomena with which we are 

 about to deal go deep into the subject of colloid chemistry, and 

 especially with that branch of the science which deals with the 

 properties of colloids in connection with capillary or surface 

 phenomena. It is to the special student of colloid chemistry that 

 we must ultimately and chiefly look for the elucidation of our 

 problem*. 



In the first and simplest part of our subject, the essential 

 problem is the problem of crystalhsation in presence of colloids. 

 In the cells of plants, true crystals are found in comparative 

 abundance, and they consist, in the great majority of cases, of 

 calcium oxalate. In the stem and root of the rhubarb, for instance, 

 in the leaf-stalk of Begonia, and in countless other cases, sometimes 

 within the cell, sometimes in the substance of the cell- wall, we 

 find large and well- formed crystals of this salt ; their varieties of 

 form, which are extremely numerous, are simply the crystalHne 

 forms proper to the salt itself, and belong to the two systems, 

 cubic and monoclinic, in one or other of which, according to 

 the amount of water of crystalhsation, this salt is known to 

 crystallise. When calcium oxalate crystallises according to the 

 latter system (as it does when its molecule is combined with two 

 molecules of water of crystalhsation), the microscopic crystals 

 have the form of fine needles, or "raphides," such as are very 

 common in plants ; and it has been found that these are artificially 

 produced when the salt is crystallised out in presence of glucose 

 or of dextrin f. 



Calcium carbonate, on the other hand, when it occurs in plant- 

 cells (as it does abundantly, for instance in the "cystoliths" of the 

 Urticaceae and Acanthaceae, and in great quantities in Melobesia 



* There is much information regarding the chemical composition and minera- 

 logical structure of shells and other organic products in H. C. Sorby's Presidential 

 Address to the Geological Society (Proc. Geol. Soc. 1879, pp. 56-93); but Sorby 

 failed to recognise that association with "organic" matter, or with colloid matter 

 whether living or dead, introduced a new series of purely physical phenomena. 



t Vesque, Ann. des Sc. Nat. (Bot.) (5), xix, p. 310, 1874. 



