60 



ON THE PLANT-CELL. 



surrounding parenchyma (fig. 47.). Still, examples are very frequent in 

 which the vascular bundles pass into the surrounding parenchyma. 



/3. In the earliest stages the vascular bundles of the Dicotyledons 

 cannot be distinguished from those of Monocotyledons. The difference 

 is first visible towards the close of the first period of vegetation. At 

 this period the aspect of the cambium is unchanged, its formative ac- 

 tivity continues, and new cells are deposited upon the vascular bundles. 

 The first part of the vascular bundle is formed under exactly the same 

 circumstances as in Monocotyledons, and exhibit precisely the same 

 appearances (fig. 48). From this point, however, the further development 



is very different, for here it is important to observe that all longitudinal 

 extension of the parts of the plant ceases. When this takes place, the 

 new-formed cells frequently continue to extend, so that they have not 

 sufficient room ; and the consequence is, that the ends of the cells in a 

 horizontal layer press themselves between the ends of the cells which 

 lie above them and below them, and thus they all become pointed. In 

 all recently formed wood-cells it may be remarked that they are shorter 

 than the old cells, and their ends are rounded. The peculiar form of 

 the prosenchyma-cells is produced later. In the first part of the vascular 

 bundles no such cells are ever found ; the innermost are longitudinal 

 parenchyma-cells, and pass gradually outwards into the wood-cells. But 

 there are instances where no such extension of the recently formed cells 

 ever takes place, and then the entire wood consists only of parenchy- 

 matous cells, as, for instance, in Bombax pentandra, Carolinea minor, 

 and perhaps all Bombacece. In the later products of the formative acti- 

 vity of the cambium, a great difference in its growth is observed, 

 according as the cells are developed as wood-cells (prosenchyma) or as 

 they are uniformly or irregularly deposited. The simplest kinds of wood, 

 on the one side, are those in which all the cells are similarly developed, 

 and where no distinction between the cells and the so-called vessels 

 exists. Such wood is seen in the Coniferce and Cycadacece, consisting of 

 lengthened prosenchyma, like broad cells with from one to eight rows of 

 pores (fig. 49. A, B). 



48 Successive unclosed vascular bundles from Vicia Faba ; a longitudinal section. 

 The arrow denotes the direction from the pith to the bark, a, Cambium cells. 



