Vegetable Physiology. 407 



much the same position as dextrine in reference to development ; 

 they occur in similar situations dissolved in the watery cell-sap, 

 abounding often in young shoots, succulent stems, &c., especially 

 at the epoch just previous to flowering. Sugar is formed in com- 

 pany with or probably from dextrine, out of the starch of germi- 

 nating tubers and seeds ; so that it is not merely a product of 

 simple assimilation, but, like dextrine, also a soluble material 

 capable of being derived from decomposition of more highly ela- 

 borated matter (starch and cellulose) where this is required for 

 new development 



We have spoken of dextrine and sugar as dissolved consti- 

 tuents of the watery cell-sap, and this is their general condition. 

 When sugar is very abundantly formed, it is sometimes deposited 

 in crystals, forming a kind of excretion, as is observed in parts 

 (nectaries) of various flowers. Tliis phenomenon, however, is 

 of minor importance physiologically speaking, and is of far less 

 interest than the occurrence of solid gummy matters, approach- 

 ing very closely in character to dextrine, and at the same time 

 distinctly related to cellulose. Gum arable, the gum of plum and 

 cherry trees, are exudations from the stems of trees, ordinarily 

 regarded as excretions resulting mostly from disease. They are 

 supposed to be derived from the watery cell-sap ; but this is very 

 doubtful, for it has been shown that gum tragacanth, which 

 exudes from various species of Astragalus, is essentially analo- 

 gous in its nature to those semi-gelatinous thickening layers of 

 the cells of seeds described in an earlier page, only that the 

 approach is here to dextrine and not to starch. In the traga- 

 canth-plants the parenchymatous cells of the pith and the me- 

 dullary rays have their walls greatly thickened as they grow old 

 with the " soft" cellulose compound, which in time loses to a great 

 extent its laminated character, and, undergoing a chemical modi- 

 fication, becomes almost homogeneous, acquiring at the same 

 time the property of swelling up strongly when placed in water ; 

 so that the access of wet to the stems causes these cells to swell 

 up, burst, and exude in a gummy mass upon the surface of the 

 bark. The gummy matter obtained from quince-seeds and linseed 

 consists in like manner of the soft thickening layers of cells, that 

 is of cellulose approaching in physical condition to dextrine, 

 retaining its structure (fig. 4) until a late stage of its existence, 

 but passing chemically into a condition in which sulphuric acid 

 and iodine do not readily produce a blue colour. In tragacanth 

 gum treated with those reagents, fragments of cell-structure are 

 revealed in the gelatinized mass ; the mucilage layers of quince 

 seeds turn blue with a more active iodized preparation (iodide of 

 zinc). The slimy substance of the Confervoid Algae, and the 

 gelatinous tissue of the larger sea-weeds, appear to consist of a 



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