278 VEGETABLE PHYSIOLOGY 
are products of respiration can be specified. Under abnormal 
conditions volatile compounds of ammonia or ammonia 
itself may be added to these. But there are 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 
Fic. 127. — Crys- 
TALS OF C'aLcluM 
OXALATE IN 
7 
Fiq. 126.—Drverorment or LystarNous Gnanp IN Watt oF CELL 
Srem oF Hypericum. Tue Four Fiqures REPRESENT oF THE Bast oF 
Sucorsstve Stages. xX 250. Ephedra. 
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 of a subsidiary water-absorbing apparatus, 
as will be mentioned 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 
