272 



VEGETABLE PHYSIOLOGY 



certain other substances which are thrown off by a few 

 plants, and may in them perhaps be regarded rather as 

 secretions, as some of them subserve definite purposes. 

 Perhaps the most frequently occurring instance of these 

 is the sugary solution known as the nectar, which is so 

 common in flowers, and which is poured out usually to 

 serve as an attraction to insect visitors. Mineral matters 

 such as calcium carbonate are in some cases excreted on to 

 the surface of the leaf, sometimes by special glands, as in 

 certain Saxifrages. In these the salt aids in the formation 



FIG. 126. DEVELOPMENT OF LYSIGENOUS GLAND IN 

 STEM OF Hypericum. THE FOUR FIGURES REPRE- 

 SENT SUCCESSIVE STAGES. X 250. 



FIG. 127. CRYS- 

 TALS OF CALCIUM 



OXALATE IN 



WALL OF CELL 

 OF THE BAST OF 

 EpTiedra. 



of a subsidiary water-absorbing apparatus, as will be men- 

 tioned in a subsequent chapter. 



In most cases the materials which we are discussing 

 are not thrown off from the plant, but are removed to parts 

 which are not concerned in the vital processes to any very 

 great extent. Ethereal oils are found deposited in special 

 cavities in leaves, stems, and other parts (fig. 126). 

 Mineral matters are often deposited in the substance of 

 cell-walls. The oxalate of calcium occurs frequently in 

 this situation (fig. 127). In other cases it is deposited in 

 special cells, where it forms clusters of crystals of eharac- 



