90 ON THE PLANT-CELL. 



tion, but much more evidence to prove that this process takes place by a 

 true intus-susception. Schwann has made some highly ingenious re- 

 searches on this subject.* 



38. At a definite period, the cell-membrane ceases for the most 

 part, or entirely, to grow ; and the assimilated matters, which are so 

 formed that they pass readily into a solid form, distribute themselves 

 in a special layer upon the inner surface of the membrane, in the 

 various forms we have already spoken of ( 16.). This process goes 

 on as long as new matters are formed. 



In the formation of crystals, we find that the constantly increasing 

 layers are deposited only of a definite thickness, and when this thickness 

 is reached the formation of a new layer begins. We find the same taking 

 place in the plant-cell ; only with this difference, that in the cell the solu- 

 tion is in the interior, and the newer layers are deposited from within. 

 Of the cause which gives to these deposits a spiral form we know little or 

 nothing ; only this much we can say, that neither in round nor longitudinal 

 isolated cells are either deposit-layers or a spiral arrangement exhibited. 

 The first indication of a spiral direction of parts is seen in the species of 

 Spirogyra ; but here the spirally deposited matter is not the formative 

 matter of the cell, but chlorophyll. 



It often happens that the primary cell-membrane continues to grow 

 after the second layer is deposited, which results in a division of the last 

 layer if it has not grown equally with the first. When a new layer con- 

 sists of another modification of the assimilated matter, or the first layer 

 becomes dry and firm before the second is deposited, a greater or less 

 evident separation between them is visible. 



39. The matters contained in the cell serve not only for the 

 completion of the cell itself, or for the formation of new cells 

 ( 13.), but also constitute, in various conditions of aggregation, 

 and under multifarious forms, the contents of the cells. In the 

 organic substances the fluid portion is very gradually transformed 

 into a relatively speaking firm, but not completely solid, matter. 

 The unazotised compounds, gum, dextrin, jelly, amyloid, starch, 

 &c., are rendered firm by the gradual abstraction of the solvent 

 (water), and, in a similar way, from the azotised compounds, is 

 formed the mucus. In consequence of this process of change, 

 many of these substances appear in remarkably defined forms, 

 requiring especial notice. Besides crystals of inorganic salts, we 

 observe, in the cells, starch-, inulin-, and mucus-granules, larger 

 globules of gum, resin, and oil. But the most remarkable of these 

 forms is one of a peculiar character, assumed by the mucus in cer- 

 tain cells of the antheridia in the Characece, Mosses, Lichens, and 

 Ferns, in which it presents the aspect of a spiral filament, with from 

 one to two turns and a half. 



The contents of the individual cells exhibit an endless variety, from a 

 mixture of many very different fluid and solid constituents to a single 

 nearly uniform material, either liquid or solid, occupying the whole cell. 



* Mikroskopisehe Untersuchungen, p. 229. 



