202 PART III. PHYSIOLOGY. [ 45 



plant, whereas others are thrown off or excreted. The nitrogenous 

 waste -products are deposited either in cells or in the laticiferous 

 tissue : there is practically no excretion of such waste-products by 

 plants. Similarly, those of the non-nitrogenous waste-products 

 which are not gaseous at ordinary temperatures, are retained by 

 the plant. For instance, oxalic acid is deposited in the form of 

 crystals of calcium oxalate either in the cavities or in the walls of 

 the cells (see pp. 78, 81) : the crystals may have either six mole- 

 cules of water of crystallisation, when they are quadratic ; or 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. 

 97) : the tannins are mostly stored in cells, dissolved in the cell- 

 sap. 



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 with 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. 78), or 

 is excreted by means of the chalk-glands (p. 96). 



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. 26), of flowers, and the di- 

 gestive liquid poured out by the glands of the insectivorous 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 insectivorous 

 plants results in the digestion of the entrapped insects (see p. 189). 



The mechanism of excretion may be generally illustrated by 

 reference to two cases : to the nectaries, and to the chalk-glands. 

 The former afford an example of that mode of excretion in which 

 the necessary force is supplied by the excreting cells themselves : 

 the latter, of that mode in which the necessary force is derived 

 from another source. Excretion by nectaries can be well observed 

 in the case oiFritillaria imperialis (Fritillary, or Crown Imperial). 

 At the base of each of the petals of the flower, there is an 

 oval depression which is the gland or nectary and is seen to be 

 occupied by a large drop of nectar. If the flower be cut off, and 

 the drop be removed from the nectary by means of blotting-paper, 

 it will be shortly replaced by a fresh drop. It is therefore clear 



