Vegetable Physwlogy. 389 



between the nucleus and the cell-Avall. As the cell- wall expands, 

 it departs further and further from the nucleus, which never 

 increases much in size ; meanwhile more protoplasm is formed 

 to fill up the intervening space, and this continues up to a certain 

 point. Then a new phenomenon presents itself; in the proto- 

 plasm appear lighter spaces, cavities filled with a thinner fluid, 

 which thus look like " bubbles" in the protoplasm ; sometimes 

 these increase in number as well as size, and give the protoplasm 

 a frothed appearance ; in other cases only one large cavity is 

 formed, which gradually and equably enlarges. When many of 

 the water-spaces exist, as they expand they coalesce moi'e or less ; 

 but remnants of the thick protoplasm forming the boundaries of 

 the spaces remain as viscid threads stretching across the central 

 cavity ; in this v/ay are formed the threads by which the nuclei 

 are often suspended ; in the other cases the nucleus is carried out 

 with the main body of the protoplasm, Avhich by the expansion 

 of the water-space is pushed out, and always adhering to the 

 cell-wall, forms a thicker or thinner layer investing this, pre- 

 senting the same character as the lining layer of the Confervac, 

 the central space, occupied by watery-cell sap, forming the great 

 cavity of the cell. The protoplasm is, however, much increased 

 in quantity during the expansion of the cells of succulent 

 parenchymatous tissues destined to form the seat of nutrition, 

 especially in leaves and other green parts, also in cells which are 

 destined to produce starch. In leaves we find a rather thick 

 layer applied against the cell-wall (fig. 9), in which it is not 

 easy to ascertain whether a distinct (formative) layer lies imme- 

 diately upon the cell-wall ; the protoplasm is of denser con- 

 sistence near the cell-wall, and when contracted by re-agents 

 presents a smooth, even surface where it was in contact with the 

 inembrane. In some of the Cellular plants the character of the 

 protoplasmic layer is more distinctly seen, and it there presents 

 an appearance of distinct layers, of different consistence, the 

 densest next the cell-wall. The layer immediately applied to 

 the wall seems to be efficient in cell-formation, while the inner 

 thicker layer is more devoted to the nutrition, judging from the 

 chlorophyll-granules which lie imbedded in it. In the Cliara 

 the circulating substance is a third stratum, lying inside that 

 layer in which the chlorophyll-granules are imbedded. In the 

 cells of the leaves of Vallisneria, the chlorophyll-granules are 

 imbedded in a rather thin layer of transparent protoplasm, which 

 extends as a sheet over the inside of the cell-wall, and "circulation" 

 observed in the cells consists in the travelling round of the 

 entire stratum of protoplasm, carrying along the chlorophyll- 

 granules and the nucleus with it ; by which indeed the motion is 

 rendered visible. 



VOL. XVII r. 2 D 



