646 ON CONCRETIONS, SPICULES [ch. 



which deals with colloids in connection with surface phenomena. 

 It is to the special student of the chemistry and physics of the 

 colloids that we must look for the elucidation of our problem*. 



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

 is the problem of crystallisation in presence of colloids. In the cells 

 of plants true crystals are found in comparative abundance, and 

 consist, in the 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 crystalline forms proper to the salt itself, 

 and belong to the two systems, cubic and monocHnic, in one or other 

 of which, according to the amount of water of crystallisation, this 

 salt is known to crystalhse. When calcium oxalate crystalhses 

 according to the latter system (as it does when its molecule is com- 

 bined with two molecules of water), the microscopic crystals have 

 the form of fine needles, or "raphides"; these are very common in 

 plants, and may be artificially produced when the salt is crystallised 

 out in presence of glucose or of dextrinf . 



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 

 and the other calcareous or "stony" algae), appears in the form 

 of fine rounded granules, whose inherent crystalline structure is 

 only revealed (like that of a molluscan shell) under polarised light. 

 Among animals, a skeleton of carbonate of lime occurs under a 

 multitude of forms, of which we need only mention a few of the 

 most conspicuous. The spicules of the calcareous sponges are 

 triradiate, occasionally quadriradiate, bodies, with pointed rays, not 

 crystalline in outward form but with a definitely crystalline internal 



* 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. G'eol. 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. 



f Jidien \'esque, Sur la production artificielle de cristaux d"oxalate de chaux 

 semblables a ceux qui se forment dans les plantcs. Ann. Sc. Xat. (Bot.) (5) xix. 

 pp. 300-313. 1874. 



