504 PHYSIOLOGY. 



not clear whether it occurs dissolved in the cell-sap or in granules 

 mixed with the protoplasm. From solutions it crystallizes in 

 spherical masses of radiating crystals, which may also be seen by 

 dipping the sections of the tissue in alcohol. As it has no special 

 reactions giving distinct colour, like starch, it cannot be detected 

 except by chemical analysis. 



Fixed Oils. The fixed oils, which occur abundantly in many 

 seeds and fruits, are easily distinguished in the cell-contents on 

 account of their forming isolated globules, merely suspended in the 

 watery cell-sap, which strongly refract light, and can be made to 

 run together into large globules by pressure and by the application 

 of ether. 



The oil-globules occur mostly in organs prepared for a season of rest, 

 as in the endosperm (Cocoa-nut) or cotyledons (Almond) of seeds, or in 

 the pericarp (Olive) of the higher plants also sometimes in tubers, as in 

 those of Cyperus esculentus. Among the lower plants oil is especially 

 abundant in the resting-spores of the Algae, taking the place of the starch- 

 granules existing during active vegetation. 



Essential Oils. Essential oils are readily distinguishable when 

 they exist in quantity suspended in the cell-sap, or entirely filling 

 the cell ; sometimes, however, they exist in such small proportions 

 as to be undistinguishable, as is the case in many scented petals. 



The essential oils are developed, like the fluid colouring-matters, in 

 vacuoles of the protoplasm, resolved in time into one large cell-cavity 

 bounded by the layer of the protoplasm lining the primordial utricle. 

 The oily matters, caoutchouc, resins, &c., are usually found in compound 

 cellular organs, glands, ducts, &c., to be mentioned presently, under the 

 head of Tissues. 



Sugar, Gum, etc. Sugar, dextrine, gum, and similar substances 

 dissolved in the watery cell-sap are not capable of detection by the 

 microscope, since the quantities in which they exist are too small 

 to alter sufficiently the refractive power of the liquids ; and we 

 have no colour-test for them. 



The gummy matters of plants (which swell up in cold water and form 

 a slimy mass) are in many cases parts of the cellulose tissues themselves, 

 as is the case in the seed-coat of Linseed, the Quince, &c. , and the gum 

 of Tragacanth, which latter consists of the collenchymotous tissue into 

 which the pith and medullary rays of the stem are gradually converted. 

 They result from the abundant formation of secondary layers in that state 

 of the " cellulose " compound which is intermediate between cell-mem- 

 brane and dextrine, just as the " amyloid " of the secondary layers of the 

 cells of some Lichens is an intermediate condition between cellulose and 

 starch. Sassorine and Arabine are formed in a similar manner, from the 

 disorganization of the cellulose matters ; hence these materials are to be 

 looked on as excrementitious. 



