76 BOTANY FOR BEGINNERS CHAP. 



strong sulphuric acid causes the yellow colour to be replaced by 

 a deep blue. 



This reaction is characteristic of a substance termed cellulose. 

 We may consequently conclude that the cell-wall consists 

 principally of such cellulose. Cellulose consists of three 

 chemical elements, known as carbon, hydrogen, and oxygen. 

 All organic bodies which have the hydrogen and oxygen 

 present in that proportion in which these elements are present 

 to form water are grouped together as carbohydrates. The 

 proportion by weight in which carbon, hydrogen, and oxygen 

 are present in cellulose is represented by the following percentage 

 composition : 



Carbon 44-44 



Hydrogen . 6' 17 



Oxygen 49'38 



99.99 (= 100 very nearly.) 



From this it will be seen that there is eight times as much 

 oxygen as hydrogen by weight in cellulose. 



But water is made up of eight parts by weight of oxygen to 

 one of hydrogen, as the following analysis shows : 



Hydrogen 11*136 



Oxygen 88*864 



lOO'OOO 



Hence, we are justified in classing cellulose as a 'carbo- 

 hydrate. 



Mineral substances such as silica, carbonate of lime, and 

 compounds of iron are also found deposited in cell-walls. 



Chemical Changes which the Cell-wall may under- 

 go. i. A portion, or all, of the cell-wall may become cuti- 

 cularised. This is caused either by a change in the 

 cellulose, or by the deposition of cutin in the cell-wall. 



The epidermal cells of some leaves afford a good example of 

 cuticularisation. If a section of a leaf of the Rhododendron be 

 touched with iodine and sulphuric acid, some of the layers of 

 the wall of the epidermal cells will assume a deep blue colour ; 

 the colour is deepest in the inner layers, the outer layers not 



