STRUCTURE OF THE CELL 



687 



varies according to the direction in which the section is made. This 

 is well shown in the pith of the young shoots of elder, lilac, or other 

 rapidly growing trees, the cells of which, when cut transversely, gene- 

 rally exhibit circular outlines ; whilst, when the section is made verti- 

 cally, their borders are straight, so as to make them appear like 

 cubes or elongated prisms, as in fig. 524. A very good example of 

 such a cellular pareiichyme is to be found in the substance known as 

 ' rice-paper,' which is made by cutting the herbaceous stem of a 

 Chinese plant termed Aralia pavyrifera, vertically round and round 

 with a long sharp knife, so that its tissues may be (as it were) unrolled 

 in a sheet. The shape of its cells when thus prepared is irregularly 

 prismatic, as shown in fig. 525, B ; but if the stem be cut transversely, 

 their outlines are seen to be circular or nearly so (A). When, as 

 often happens, the cells have a very elongated form, this elongation 

 is in the direction of their grow r th, 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, whose cells are greatly elongated in the horizontal 

 direction (fig. 547, ), 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 w T hich they will have to pass through the least number of parti- 

 tions ; 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 pre- 

 sent is the stellate cell, repre- 

 sented in fig. 526, forming the 

 spongy parenchymatous substance 

 in the stems of many aquatic 

 plants, of the rush for example, 

 which are furnished with air- 

 spaces. In other instances these 

 air-spaces are large cavities which 

 are altogether left void of tissue : 

 such is the case in NupTiar Itttfft 

 (the yellow water-lily), the foot- 

 stalks of whose leaves contain large air-chambers, the? walls of 

 which are built up of very regular cubical cells, whilst some curiously 

 formed large stellate cells project into the cavity which they bound 

 (fig. 527). The dimensions of the component vesicles of cellular tissue 

 are extremely variable ; for although their diameter is very com- 

 monly between ^^th and ^th of an inch, they occasionally mea- 

 sure as much as -j^th of an inch across, whilst in other instances 

 they are not more than ^^th, 



The cells of a growing tissue are always formed, as we have seen, 

 by cell-division, that is, by the formation of cellulose walls across 

 cells previously in existence. The original cell-wall must therefore 

 always be single. It is only in older thick-walled cells that a line of 



FIG. 526. Section of stellate 

 parenchyme of rush. 



