246 LECTURE XII. 



The excretion of salts in solution (principally calcium 

 and magnesium carbonates) is most commonly effected by 

 means of a well-developed gland. A gland of this kind, 

 from the leaf of Saxifraga crustata, is shewn in Fig. 19 

 (p. 91). The gland consists of a group of modified meso- 

 phyll-cells in connexion with the termination of a fibro- 

 vascular bundle: one or two water-pores are present in the 

 epidermis immediately over it. We have seen that, under 

 the action of the root-pressure, the gland excretes water. 

 The excreted water collects in the depression at the margin 

 of the leaf at the bottom of which the gland lies, and as the 

 water evaporates the salts which it holds in solution are left 

 behind in the solid form. This solid residue is prevented, 

 for a time at least, from filling up the water-pores (Fig. 19, b), 

 which are present upon the surface of the pit and upon 

 which the salts are especially deposited. The margin of the 

 leaf of this plant is marked by a series of mineral aggrega- 

 tions formed in this way, resembling beads, each of which 

 corresponds to a gland. Glands of this kind are commonly 

 present, though they are not so well-developed, in many 

 allied plants belonging to the Saxifragacese and Crassulaceae. 



In other cases these salts appear to be excreted by 

 ordinary epidermal cells. In certain Ferns (various species 

 of Polypodium and Aspidium) scales of calcium carbonate 

 are formed on the surface of depressions in the surface which 

 are situated immediately over the terminations of fibro- 

 vascular bundles. Similar scales occur also on the leaves 

 and herbaceous stems of various Plumbaginaceous plants, 

 but in these they bear no relation to the fibrovascular 

 bundles. In these cases no glands, like that described above, 

 are present; it is therefore to be concluded that the epi- 

 dermal cells themselves excrete the calcium carbonate. 



It not unfrequently happens, however, that plants excrete 

 substances other than waste-products, but this has the effect 

 of securing indirect advantage to the plant. In the great 

 majority of flowers there are glandular organs which excrete 

 a watery fluid holding principally sugar in solution ; the 

 organs are termed nectaries, and the excretion nectar. A 



