FORM OF THE PLANT-CELL. 57 



bundles, later the parenchyma, and at last there remains a portion of 

 cambium at the point of the stem (Punctum vegetationis, C. Fr. Wolff), 

 and externally to the vascular bundles. This last part has been more 

 especially characterised as cambium, but it does not differ from the other. 

 The cambium is not an unorganised mass, as was formerly supposed, but 

 in the vascular plants, at least, is always a cellular tissue containing cyto- 

 blasts, and, in a state of active vitality, forming new cells, a part of 

 which adheres in all its forms to the cellular tissue already formed, and 

 a part remains as cambium to carry on the process of growth. In this 

 cambium the following tissues originate : 



The Vascular Bundles. A large series of observations prove that 

 the vessels, and to a certain extent the cells connected with them, cease 

 to exhibit the collective energy of cell-life sooner than the neighbouring 

 cells. They cease earlier to develope new cells, they pass sooner from 

 the condition of the general nutrition of the membrane into that of the 

 deposition of secondary layers, they consume quicker their assimilated 

 contents without forming new ones, and when the neighbouring cells 

 are first commencing their chemical activity they have either consumed 

 all their juices, or convey only air (the vessels) or a very homogeneous 

 indifferent sap (the young wood-cells). There are cells which pass 

 through all the stages of life quicker than the parenchyma-cells. In 

 this way all the phenomena may be easily and perfectly explained. The 

 parenchyma-cells form new cells when the vascular bundles have ceased 

 to do so. There will, therefore, be always present, in a longitudinal 

 mass, a larger number of parenchyma-cells than cells of the vascular 

 bundles ; the last are always much longer than the first. This antago- 

 nism is especially evident at the commencement of a vascular bundle, 

 at least at its sides, where its cells gradually pass into the parenchyma. 

 As the formation of the secondary layers is an important point in the 

 permanent development of cells, so the form of individual cells of the 

 vascular bundles depends on the period in which they originate. Several 

 kinds of vascular bundles are recognised. 



1. In the higher Cryptogamia, the Ferns, Lycopodiacece, Equisetacece 

 (cryptogamic vascular plants), sometimes in aerial stems (less in the 

 creeping subterranean stems, and in the Equiseta generally), the entire 

 vascular bundle arises and is developed at the same time. In these 

 vascular bundles there is a great similarity of form ; and, as the stems of 

 these plants increase little in length after the formation of the vascular 

 bundles, almost the only kind of vessel seen is that with cleft-like pores.* 

 The Lycopodiacece have vessels with a very narrow spiral fibre ; the 

 Equisetacece f annular vessels, but with narrow rings.J 



2. In the Phanerogamia a successive formation of vascular bundles 

 takes place. The parts next the axis are first developed from the cam- 

 bium, and the development extends gradually towards the periphery. 

 Then appear the parts belonging to the vascular bundles, but never 

 until the other portions have made considerable progress. From this 

 arises several important modifications of the vascular bundles. The 

 type of the deposit layer depends on the nature of the vessel at first. 

 Nearest the axis we find distant annular vessels ; following these, vessels 



* Mohl de Structura Caudicis Filicum arborescent! um. Munich, 1833. 



{ They ought to he placed highest among the Cryptoyamia, according to my view. 



j Bischoflf, Die kryptogamischen Gewachse. Niirub. 1828. 



