of the Stem of the Phanerogamia. 391 



and forming a uniform tissue, in which we can find no trace of 

 the subsequent separation into parenchyma, vascular bundles, 

 &c. We may term this, with Schacht, the primary parenchyma 

 {Urparenchyma) : the restriction of the term cambium to this 

 tissue would be very appropriate, were it not that this name has 

 long been generally applied to the structures of the second pe- 

 riod, and especially to the still more developed cambium-layer 

 of Dicotyledonous trees. In the second period the tissue sepa- 

 rates into the different classes of tissues, the formation of paren- 

 chymatous cells being continued by the division of a portion of 

 the cells in different directions of space; while in other groups 

 of cells the formation of elongated cells, vessels, &c, results 

 from the predominance of longitudinal division, and the omis- 

 sion, or rare occurrence, of cross-division. In this period also 

 the tissue has very delicate cell-walls, and, from the absence of 

 intercellular passages, is still transparent. The cell-walls of this 

 and the preceding period have the peculiarity, in contrast to the 

 walls of more developed cells, that they attract the pigment from 

 a solution of carmine, and become tinged bright red. The com- 

 mencement of the third period, in which the tissue emerges from 

 the condition of cambium, is marked in the parenchyma by the 

 appearance of intercellular passages, which become filled with 

 air, and in the elongated cells, vascular tubes, &c, by the de- 

 position of secondary layers. The longitudinal division of the 

 parenchymatous cells in the direction parallel with the surface 

 of the axis has now pretty well ceased ; but the transverse divi- 

 sion and the multiplication of cells lengthwise of the axis lasts 

 for a considerable time longer in many cases. This allows the 

 possibility of particular parts of a tissue, thus tolerably advanced 

 in development, and even arrived at complete conformation, re- 

 curring again to the condition of cambium, producing within 

 them a young tissue by cell-division, thus giving rise to the 

 formation of vascular bundles, peridermal layers, &c. 



Passing from the consideration of the cambium to Schleiden's 

 account of the development of the stem, the following five cases 

 are described*. 



a. The first distinction occurs between Monocotyledons and 

 Dicotyledons, in the latter of which the individual vascular 

 bundles continually increase in thickness, while in the Mono- 

 cotyledons this process of cell-formation ceases from below 

 upwards, and hence thickening of the individual internodes be- 

 comes impossible, and a thickening of the axis can only arise 

 successively from the following internodes becoming constantly 

 broader. 



The Dracance, indeed, form an exception to this. 



* Grundziige, 1st ed. ii. p. 128. 



97* 



