CHAPTER II. SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 729 



two molecules, when they are prismatic (raphides). The resins 

 and ethereal oils are usually excreted by the cells in which they are 

 formed, into intercellular spaces (resin-ducts, oil-glands, see p. 138): 

 the tannins are mostly stored in cells, dissolved in the cell-sap. 



Some general idea may be formed as to the relation of these substances to 

 the catabolic processes by a consideration of the origin of oxalic acid and of 

 tannin in the plant : the matter has been more fully investigated with refer- 

 ence to these substances than to any others, and the results in these two cases 

 correspond in a remarkable manner. 



In the first place, both these substances are abundantly formed in connexion 

 with active chemical change of the plastic substances (probably the nitro- 

 genous) supplying the demands of a growing part : for instance they are 

 abundant in seedlings, in developing shoots, roots, etc. This formation, which 

 is quite independent of light, is distinguished as primary, and the calcium 

 oxalate and the tannin thus formed remain permanently in the cells in which 

 they are formed : the primary calcium oxalate is characterised by the prismatic 

 form (raphides). A secondary formation of both these substances takes place in 

 green pans of plants exposed to light, in leaves for instance : they are formed 

 so abundantly under these circumstances that they must be continually trans- 

 ported (in solution by osmosis) from the seat of formation to other parts, for 

 instance, from the leaves to the stem. This secondary formation is associated, 

 not with the assimilation of carbon dioxide, but with the formation of 

 nitrogenous organic substance (see p. 720) : but though associated with this 

 anabolic activity, the actual process of formation of these two substances is 

 probably not synthetic, but analytic ; it is a catabolic process which necessarily 

 accompanies the anabolic. 



The oxygen which is set free in connexion with the decomposi- 

 tion of C0 2 in the green parts under the influence of light, is 

 exhaled in the gaseous form ; this is also the case, for the most 

 part, with the watery vapour and the carbon dioxide produced in 

 catabolism. In some cases, however, some portion of the carbon 

 dioxide forms calcium carbonate, which is either deposited in the 

 solid form (e.g. cystoliths, see p. 108), or is excreted by means of 

 the chalk-glands (p. 137). 



In some cases, substances of nutritive value are excreted by 

 plants, as for instance, the sugary liquid known as nectar by 

 special glands, the nectaries (see p. 679), of flowers, and the 

 digestive liquid poured out by the glands of the carnivorous plants. 

 This loss of substance is, however, compensated for by the advan- 

 tages gained by the excretion. The nectar attracts insects, and so 

 ensures cross-fertilisation, and the excretion of the carnivorous 

 plants results in the digestion of the entrapped insects (see p. 711). 



The mechanism of excretion may be generally illustrated by 



