VON MOHL'S VEGETABLE CELL. 53 



which these possess. The treatise, therefore, contains, if an 

 imperfect, still in many respects a more extensive resume of 

 Vegetable Physiology than might have been conjectured from 

 the title. 



" Unhappily, the Physiology of Plants is a science which 

 yet lies in its earliest infancy. Few of its dogmas can be re- 

 garded as settled beyond doubt ; at every step we meet with 

 imperfect observations, and consequently with the most con- 

 tradictory views ; thus, for example, opinions are still quite 

 divided regarding the doctrines of the development of the 

 cell, of the origin of the embryo, and of the existence of an 

 impregnation in the higher Cryptogams. Both in these and 

 in other cases, the small compass of the present treatise for- 

 bids a more extensive detail of the researches upon which the 

 opposing views are founded ; I hope, however, that I have 

 succeeded in making clearly prominent the chief points upon 

 which these contests turn, and thus in facilitating the forma- 

 tion of a judgment by the reader; and I have never neglected 

 to indicate the literature from which further instruction is to 

 be derived." 



It may be well to notice the views of so excellent an ob- 

 server upon sundry points which have been more or less mat- 

 ters of controversy. As to the milk-vessels, or vessels of the 

 latex, Mohl inclines to adopt the view that considers them as 

 intercellular passages which have acquired membranous lin- 

 ings (p. 2). He denies that the membrane of nascent cells 

 is soluble in water, as Schleiden states (p. 9). He briefly 

 states the ground on which, in his controversy with Halting 

 and Mulder, he successfully maintains that the primary cell- 

 membrane is thickened by successive concentric layers of 

 cellulose deposited on its inner face. The combination of 

 spiral markings and pits on the wood-cells of Taxus and Tor- 

 reya, as also in the Linden, is explained by considering the 

 former to belong to a second layer or deposition within that 

 to which the pits belong. This tertiary membrane or deposit 

 forms the spiral fibre or band in the cells of the seed-coat of 

 Collomia, the hairs of the achenium of Senecio, etc. (p. 18). 

 The whole subject of spiral and other markings, rings, dots, 



