354 



THE MICROSCOPE AND ITS REVELATIONS. 



Era 



ing 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, generally exhibit circular 

 outlines; whilst, when the section is made vertically, their borders are 

 straight, so as to make them appear like cubes or elongated prisms, as in 

 Fig. 235. A very good example of such a cellular parenchyma 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 papyrifera 1 verti- 



cally 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. 236, B; but if 

 the stem be cut transversely, their 

 outlines are seen to be circular or 

 nearly so (A}. When, as often hap- 

 pens, the cells have a very elongated 

 form, this elongation is in the direc- 

 tion 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, 



vmrnol^ -nrhinVi ia afPnvrlprl hv fhp 

 lamely, WHICH IS attOK Py \ 



< medullary raVS OI JiXOgenOUS 

 , /o otvn\ 1 ^^llr, ,. r* n 



stems ( 370), whose cells are great- 

 ly elongated in the horizontal direc- 

 tion (Fig. 259, 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 Koots, 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 the stellate cell, represented in Fig. 237, form- 

 ing the spongy parenchyinatous 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 the NupUar lutea (yellow water- 

 lily), the footstalks 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. 

 238). The dimensions of the component vesicles of Cellular tissue are 

 extremely variable; for although their diameter is very commonly between 

 l-300th and l-500th of an inch, they occasionally measure as much as 

 l-30th of an inch across, whilst in other instances they are not more than 

 l-300th. 



352. The component cells of Cellular tissue are usually held together 

 by an intercellular substance, which may be considered analogous to the 



Section of Leaf of Agave, treated with dilute 

 nitric acid, showing the primordial utriclj con- 

 tracted in the interior of the cells: a, Epidermic 

 cells; 6, boundary-cells of the stoma; c, cells of 



parenchyma; d t their primordial utricles. 



1 The jEschynomene, which is sometimes named as the source of this article, 

 is an Indian plant employed for a similar purpose. 



