62 
MORPHOLOGY OF THE CELL. 
Sect. io. The Cell-sap. — The term Cell-sap may be understood in a wider 
or in a narrower sense. In the former it would express the collective mass of all 
the fluids by which the cell-wall, the protoplasm-body, and all other organised 
structures of the cell are saturated, and would also include the fluids contained in 
the vacuoles of the protoplasm ; in a narrower sense the latter only is ordinarily 
designated as cell-sap. In any case there are grounds for considering the com- 
position of the cell-sap as very variable, according as it has been imbibed by the 
protoplasm, the chlorophyll, the cell-wall, or the starch-grains of one and the same 
cell, or occurs as vacuole-fluid. The latter may in general represent the reservoir 
out of which the organised absorbent parts of the cell supply their needs, but in 
which, on the other hand, the superfluous soluble products of assimilation and 
metabolism^, and the food-materials that have been absorbed, also for a time collect. 
One constituent of the cell-sap, water, is always common to the vacuole-fluid and to 
the fluid which saturates the organised structures. The share taken by the water of 
the cell-sap in the entire building-up of the cell has already been entered into suffi- 
ciently in detail. Its function in the cell is a very manifold one ; it is at once the 
general solvent and the agent of transport of the food-materials within the cell ; the 
water itself enters in many ways into the chemical constitution of the substances 
produced in the plant; its elements are essential for the production of assimilated 
substances ; for the formation of organised structures, the cell-wall, the protoplasmic 
structures, and the starch-grains, it is indispensable (water of organisation); the 
growth of the whole cell depends immediately on the absorption of water, and on the 
accumulation of the cell-sap as vacuole-fluid (see Figs, i, 41, 42, pp. 2, 42, 43). The 
increase in size of rapidly growing cells is nearly proportional to the accumulation 
of the sap in them. The hydrostatic pressure which the vacuole-fluid exercises on 
the primordial utricle and cell-wall is a factor in determining the form of the cell. 
The substances dissolved in the water of the cell-sap — whether salts absorbed from 
without, or compounds produced in the plant itself by assimilation and metabolism — 
are, as such, not immediately the subject of morphological observation, to which we are 
for the time confining ourselves. But the cell-sap sometirties contains substances the 
presence of which in the cell in characteristic forms can be proved by simple reactions, 
or which occur in nature in the form of definite structures, as drops or granules. Among 
the most important of the former is Inulin ^. This substance, nearly related in com- 
position to starch and sugar, occurs in the cell-sap of many Compositae^. In sap 
^ [In the first edition of this translation the term ' Stoffwechsel,' which includes as a general 
term any transformations which are effected in the products of assimilation, was translated 
'metastasis.' In the literature of animal physiology the same idea has been rendered by the terms 
metabolism and metabolic (see Mayne's Expository Lexicon, i860; Foster's Text-book of Physio- 
logy). For the sake of uniformity these terms will be adopted in the following pages. The 
products of metabolic transformations have been conveniently termed. by Foster ' metabolites.'] 
^ Sachs, Bot. Zeitg. p. 77, 1S64. — Prantl, Das Inulin, ein Beitrag zur Pflanzen-Physiologie; 
Preisschrift, Munich 1870. — Dragendorff, Materialien zu einer Monographie des Inulins, Petersburg 
1870. 
3 [Kraus, Bot. Zeitg. 1875, p. 171, shows that, in addition to Compositse, inulin is found also in 
the Campanulaceae, Goodeniacese, Lobeliaceoe, and Stylidese ; and in these orders not only in the 
underground organs, but also in the stems and in the cells of the leaves which contain chlorophyll. 
Its solubility in water appears to vary ] 
