388 



VARIOUS FORMS OF CELLULAK TISSUE. 



tissue may be (as it were) unrolled in a sheet. The shape of the 

 Cells, as seen in the 'rice-paper' thus prepared, is irregularly pris- 

 matic, as shown in Fig. 198, b ; hut if the stem he cut trans- 

 versely, their outlines are seen to be circular or nearly so (a). 

 When, as often happens, the Cells have a very elongated form, this 



Fig. 198. 



Sections of Cellular Parenchyma of Aralia, or Rice-paper 

 plant :— a, transversely to the axis of the stem ; b, in the di- 

 rection of the axis. 



elongation is in the direction of their growth, which is that, of 

 course, wherein there is least resistance. Hence their greatest 

 length is nearly always in the direction of the Axis ; but there is 

 one remarkable exception, — that, namely, which is afforded by the 

 ' Medullary Rays ' of Exogenous Stems (§ 302), whose cells are 

 greatly elongated in the horizontal direction (Fig. 210, a), their 

 growth being from the centre of the stem towards its circumference. 

 It is obvious that fluids will be more readily transmitted in the 

 direction of greatest elongation, being that in which they will 

 have to pass through the least number of partitions ; and whilst 

 their ordinary course is in the direction of the length of the Roots, 

 Stems, or Branches, they will be enabled by means of the Medullary 

 Rays to find their way in the transverse direction. — One of the 

 most curious varieties of form which Vegetable cells present, is 

 that represented in Fig. 199, which constitutes the stellate cell. 

 This modification, to which we have already seen an approxima- 

 tion in Volvox (§ 195), is found in the spongy parenchymatous 

 substance where lightness is an object ; as in the Stems of many 

 Aquatic plants, the Rush for example, which are furnished with 



