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 



Fia. 126. DEVELOPMENT OF LYSIGENOUS GLAND IN 

 STEM OF Hypericum. THE FOUR FIGURES REPRESENT 

 SUCCESSIVE STAGES, x 250. 



FIG. 127. CRYS- 

 TALS OF CALCIUM 



OXALATE IN 



WALL OF CELL 

 OF THE BAST OF 

 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 



