66 GROUPS OF TISSUES, OR TISSUE SYSTEMS 
tions of the plant and of giving buoyancy to the part in 
which they occur. 
97. Another important function of tissues is that of 
storage of food substances. Storage tissues are usually 
composed of large parenchyma cells 
with large central vacuoles and compara- 
tively little protoplasm. In some special 
cases where the storage product is one 
of the hemicelluloses this is deposited 
against the cell wall forming a sort of 
Fic. 34,—Starch SClerenchyma tissue. 
ts gata: 98. In many plants are found secretory 
cells. These often line closed cavities 
(or “‘reservoirs’”’) or elongated passages. These cavities 
or passages may be formed simply by the pushing 
apart of certain cells as the secretion is poured into 
the space between them (i.e. produced schizogenously) 
or certain cells may be dissolved, forming ‘“lysigenous”’ 
cavities. Good examples of 
the first type are shown by 
the gum canals of the ivy 
(Hedera helix) and the tur- 
pentine canals of conifers or ~* 
the glands of the leaves of St. ¥16- 35.[Gum and turpentine 
John’s wort (Hypericum). 
In the leaves and fruits of Rutaceae the cavities more 
often arise by the dissolving of the secretory cells thus 
setting free the secretion within a cavity. The secretions 
are usually gums or ethereal oils, often containing resins, 
etc. Other cells containing crystals of calcium oxalate 
and other substances, perhaps including tannin, may 
possibly be classed as excretory organs in which the 
excretions are stored up in the absence of any structure 
that would permit their being thrown out of the plant. 
