COMPOSITION OF PLANTS 2Q 



stem, but it is more compact than the fibrous tissue of the 

 bark and cannot be as readily separated into strands. 



62. If twigs two and three years old are cut across we find 

 that there are differences besides merely that of thickness. In 

 the older stems the surface of the bark has changed color, 

 become firmer and also perhaps rougher. There is less, if any, 

 evidence of chlorophyll, and the bark is thicker. The pith 

 shows little change, but the woody cylinder is about twice or 

 three times as thick as before and is divided by concentric 

 circles into annual rings of growth. Crossing these circles of 

 growth at right angles are narrow radial lines of pith which 

 connect the central pith core with the bark. These are the 

 medullary rays. 



Composition of Plants 



63. It is evident that water constitutes a very large per 

 cent, of the substance of plants. If a portion of plant tissue be 

 weighed and then subjected to a moderately high temperature 

 until it is thoroughly dried and then weighed again, it will be 

 found to have lost from 50 to 95 per cent, of its weight. In suc- 

 culent herbs the percentage of water is very great, while in 

 woody tissues it is much less. A moment's thought will show 

 that the water contained in plants must be absorbed chiefly by 

 the roots, for plants may grow and flourish even though water 

 never falls upon the stem and leaves. 



64. If after thoroughly drying vegetable tissue the tempera- 

 ture be increased to just short of the point of ignition the tissue 

 becomes black and there finally remains only a mass of charcoal 

 (carbon), equal in weight to about 25 per cent, of the dried mass. 

 During the process of charring various vapors and gases are 

 driven off; among others are the vapor of water (H 2 O), carbon 

 dioxide (C0 2 ), carbon monoxide (CO), marsh gas (CH 4 ) and 

 other hydro-carbons. After complete ignition of the charcoal 



