THE PLANT-CELL. 17 



accounted for these appearances by ascribing them to an 

 unequal distribution of water and of solid matter in the cell- 

 wall, some parts containing a larger proportion of solid 

 matter, others a larger proportion of water. On this view the 

 striae mentioned above are the expression of the alternation 

 of more and less dense layers in planes inclined to its long 

 axis, and the concentric layers of the alternation of more or 

 less dense layers from within outwards. Dippel and Stras- 

 burger explain these appearances in altogether a different 

 way. They have come to the conclusion that a cell-wall, or 

 rather each of the concentric layers of a thickened cell-wall, 

 consists of a number of spirally- wound bands, and that the 

 striae of the cell-wall are the planes of contact of these 

 bands. With regard to the stratification, if the growth in 

 thickness of the cell-wall by apposition be assumed, the con- 

 centric layers correspond to successive laminae. 



It may be stated generally that the growth in thickness 

 of cell-walls is accompanied by changes in their physical 

 properties, or in their chemical composition, or in both. The 

 formation of mucilage and of gum, for instance, depends 

 upon an alteration of the cell-wall which increases its capacity 

 for absorbing water, and this is so great in certain cases that 

 the cell-wall becomes actually soluble : but this modification 

 of its physical properties is not accompanied by any change 

 in the ultimate chemical composition of cellulose, though 

 doubtless by an alteration in its molecular constitution. 



It must not be assumed, however, that a cell-wall always consists of 

 cellulose at its first formation ; it may consist from the beginning of a 

 substance other than cellulose. This may be observed, for instance, in 

 the extine of some pollen-grains, the exospore of spores, which give from 

 the first the reactions of cuticle. 



Mucilage is found in various parts of plants ; in seeds, notably in those 

 of the Quince and in Linseed; in the roots of the Marsh-Mallow; in the 

 fruits, stems, and leaves of various plants. In seeds it is derived from 

 the middle layers of the thickened walls of the epidermal cells. The 

 various kinds of Gum are produced by the alteration of the whole wall 

 of the parenchymatous cells which form the pith and medullary rays of 

 certain plants, most of which belong to the Leguminosae and Rosacese. 



Frank considers that mucilage is not a definite chemical substance, 

 but in some cases a form of cellulose, in others a form of gum ; he is 



V. 2 



