132 



VEGETABLE LIFE AND WORK. [SECTION 16, 



441 



much thickened and the calibre small. The protoplasm of each living cell 

 appears to be completely shut up and isolated in its shell of cellulose ; but 

 microscopic investigation has brought to view, in many cases, minute 

 threads of protoplasm which here and there traverse 

 the cell-wall through minute pores, thus connecting 

 the living portion of one cell with that of adjacent 

 cells. (See Fig. 447, &c.) 



403. The hairs of plants are cells formed on the 

 surface ; either elongated single cells 

 (like the root-hairs of Fig. 441, 442), 

 or a row of shorter cells. Cotton 

 fibres are long and simple cells grow- 

 ing from the surface of the seed. 



404. The size of the cells of which 

 common plants are made up varies 

 from about the thirtieth to the thou- 

 sandth of an inch in diameter. An 

 ordinary size of short or roundish 

 cells is from -g^y to yj-j- of an inch ; so that there may generally be from 

 27 to 125 millions of cells in the compass of a cubic inch ! 



405. Some parts are built up as a compact structure ; in others cells 

 are arranged so as to build up regular air- 



channels, as in the stems of aquatic and other 

 water-loving plants (Fig. 440), or to leave 

 irregular spaces, as in the lower part of most 

 leaves, where the cells only here and there 

 come into close contact (Fig. 443). 



406. All such soft cellular tissue, like 

 this of leaves, that of pith, and of the green 

 bark, is called PARENCHYMA, while fibrous 

 and woody parts are composed of PBOSEN- 

 CHYMA, that is, of peculiarly transformed 



407. Strengthening Cells. Common cellular tissue, which makes UT> 

 the whole structure of all very young plants, and the whole of Mosses 

 and other vegetables of the lowest grade, even when full grown, is too 

 tender or too brittle to give needful strength and toughness for plants 

 which are to rise to any considerable height and support themselves. In 

 these needful strength is imparted, and the conveyance of sap through the 

 plant is facilitated, by the change, as they are formed, of some cells into 

 thicker-walled and tougher tubes, and by the running together of some of 



Fia. 441. Much magnified small portion of young root of a seedling Maple 

 tsuch as of Fig. 82); and 442, a few cells of same more magnified. The prolonga- 

 tions from the back of some of the cells are root-hairs. 



Fia. 443. Magnified section through the thickness of a leaf of Florida Star- 

 Amse. 



