SECT. L CELLULAR TISSUE. 



173 



leaf, the white ones are those of the colourless skin. 

 Since the primordial cell is the medium in which light 

 and heat act, cellular tissue is present wherever growth 

 is in progress, for all the vital operations take place 

 within its cells. All the organs of plants in their ear- 

 liest stage consist entirely of cellular tissue, and even 

 in full grown trees the bark and pith of the stem, as 

 well as the soft parts of leaves and flowers, are gene- 

 rally composed of the cells of this tissue, which though 

 assuming a great variety of forms never deviates far from 

 the original type. Every important change in the struc- 

 ture of the cell dimi- 

 nishes or destroys its 

 power of contribut- 

 ing to the nourish- 

 ment of the plant, as 

 appears in all the tis- 

 sues derived from it, 

 and which, according 



4rk M -awn IMVJil 10 8' 4< Vertical section of the cuticle of Iris ger- 



10 J1L. VOn JXLOILL, 1 ma nica :-a, cells of the cuticle ; b, cells at the sides 



a -nppPQoarv nnnP of the stomata; c, sraaU green cells placed within 



a necessary COnse- these . d opc nings of the stomata ; *, lacunse of the 



quence Of the dis- P^^hyma ; /, cells of the parenchyma. 



appearance of the vital part of the primordial cell from 

 those parts of the cellular tissue destined to undergo 

 the change. 



The fibro-vascular bundles which constitute the wood 

 of trees, and form consecutive cylinders round the stem 

 between the pith and the bark, consist of vascular ducts 

 and woody fibre. The vascular ducts are formed of 

 large wide cells each standing upon the other's flattened 

 end, their cavities being thus separated from each other 

 by septa or partitions directed at right angles to their 

 longitudinal axis. This vascular tissue when young 

 conveys the sap from the roots through the stem and 

 branches to the leaves. It forms part of the stems of 

 all climbing and quick-growing plants, in which the 



