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 
Fic. 127.— Crys- 
TALS OF CALCIUM 
OXALATE IN, 
Fic. 126.—DEvELOPMENT oF LysIGENOUS GLAND IN Wali oF CELL 
stem or Hypericum. THE FOUR FIGURES REPRE- OF THE Bast or 
SENT SUCCESSIVE STAGES. x 250. Ephedra. 
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 charac- 
