1 8 LECTURE II. 



also of opinion that gum is not necessarily a degradation-product, but 

 that it may be a normal constituent of the cell- wall. 



Kirchner has obtained from Quince seeds a substance which he 

 believes to be pure mucilage. He regards it as a compound of gum and 

 cellulose, and assigns to it the formula C 18 H, 8 O 14 . On boiling with dilute 

 hydrochloric acid it decomposes into cellulose and gum according to the 

 following equation : 



Cellulose. Gum. 



CwHaOu + H 2 O = C 6 H 10 O 5 + 2C 6 H 10 O 5 . 



The substance known as Pectose (Fre'my), which is to be found in 

 many parts of plants, more especially in unripe fruits and in bulbous 

 roots, is allied to mucilage; it is probable that this substance is also 

 derived from cellulose. 



In most cases, however, the modification of the physical 

 properties of the cell-wall is correlated with a considerable 

 alteration in its ultimate chemical composition, inasmuch as it 

 is due to the presence of substances, either organic or inor- 

 ganic, which are chemically different from cellulose. The larger 

 the proportion of these substances present, the more complete 

 is the modification of the physical properties of the cell-wall ; 

 and conversely, inasmuch as a certain proportion of cellulose 

 is always present in the chemically altered cell-wall, it always 

 exhibits in some degree the physical properties which we 

 have found to be characteristic of unaltered cell-walls. 



The organic substances which occur in altered cell-walls 

 are suberin and lignin, and cell-walls containing these sub- 

 stances are said to be cuticularised and lignified respectively. 

 These substances have a larger proportion of carbon in the 

 molecule than cellulose has. The cell-walls in which they are 

 present in any considerable quantity do not give the blue 

 colour when treated with iodine and sulphuric acid ; their 

 presence diminishes the capacity of the cell-wall for absorbing 

 water, and, in the case of cuticularised cell-walls, their optical 

 properties are modified. 



It is not yet possible to account satisfactorily for the 

 presence of these substances in cell-walls. They are probably 

 the result of the modification of the cell-wall, and not of 

 infiltration. It is not easy to cpnnect their production with 

 the metabolic activity of the protoplasm, for it has been ob- 

 served that lignification takes place, in Conifers at least, after 



