MORPHOLOGY OF THE CELL. 



concentrically-disposed layers : firsdy, an outer skin, firm and elastic, the Cell-mem- 

 brane or Cell-wall, consisting of a substance peculiar-to itself, which we call Cellulose 

 (Fig. I, B, C, h). Close up to the inner side of this entirely closed membrane is 



a second layer, also entirely 

 closed, the substance of which 

 is soft and inelastic, and always 

 contains albuminous matter ; 

 H. V. Mohl, who first dis- 

 covered this substance, has 

 given it the very distinctive 

 appellation of Protoplasm ^. 

 In the condition of cells now 

 under consideration it forms a 

 sac enclosed by the cell-wall, in 

 which usually also other por- 

 tions of protoplasm are pre- 

 sent in the form of plates and 

 threads (Fig. i, B, C,p). Ab- 

 sent from some of the lowest 

 organisms, but present in all 

 the higher plants without ex- 

 ception, there lies imbedded 

 in the protoplasm a roundish 

 body, the substance of which 

 is very similar to that of the 

 protoplasm, the Nucleus (Fig. i, 

 A, C,k). The cavity enclosed 

 by the protoplasm-sac is filled 

 with a watery fluid, the Cell- 

 sap (Fig. I, B, C, s). And 

 .besides this, there are also 

 very commonly found in the 

 interior of the cell granular 

 bodies, which however may be passed over for the moment. 



Thus, then, cells in the stage of development now described consist of a firm 

 membrane, soft protoplasm (including the nucleus), and fluid cell-sap. At first, 

 however, the sap is wanting; if the same cells be examined in a very early state 

 of their development they are smaller (Fig. i. A), their cell- wall thinner, and the 

 protoplasm forms a solid body in the middle of which lies the nucleus, at this time 

 relatively very large (k). The cell-sap first appears when the whole cell is increasing 

 quickly in volume (Fig. i, B); it presents itself originally in the form of drops 

 (vacuoli) in the interior of the protoplasmic body (Fig. i, B, s); at a later period 

 these usually coalesce, and form a single sap-cavity (Fig. i, C, s) which is enclosed 

 by the now sac-like hollow substance of the protoplasm. 



Fig. I.— Parenchyma-cells from the central cortical layer of the root oi 

 Fritillaria imperialis ; longitudinal sections ( x 550). A very young cells lying 

 close above the apex of the root, still without cell-sap. B cells of the same 

 description about 2 mm. above the apex of the root ; the cell-sap s forms separate 

 drops in the protoplasm /, behind which lie walls of protoplasm ; C cells of the 

 same description about 7—8 mm. above the apex of the root ; the two cells to the 

 right below are seen in a front view ; the large cell to the left below is in section ; 

 the cell to the right above is opened by the section ; the nucleus shows, under the 

 influence of the penetrating water, a peculiar appearance of swelling (x, y). 



^ H. V. Mohl, Ueber die Saftbewegungeii im Inneren der Zellen.— Bot. Zeitg. 1846, p. 7, 



