76

LIGNIN.

together. When we burn it, or in any other wayweaken the affinity which the elements have for eachother, they separate, and by combining together, gener-ally form water, carbonic acid gas, and ammonia.

223. When a plant is boiled in water, it is foundthat part of the plant dissolves in the water, whilstpart remains insoluble, and we are unable by long-continued boiling to make the whole of it dissolve inthe water. These, then, are two great divisions ofvegetable matter—that which is soluble in water, andthat which is not. By very simple operations of thiskind it is easy to discover that plants are composed ofa variety of different substances, readily distinguishedfrom each other by the different properties which theypossess. Of those which are usually found in all plants,the most abundant are called lignin, starch, gum,sugar, gluten, and albumen. The four former consistof carbon, oxygen, and hydrogen alone, whilst the twolatter contain, in addition to these elements, a portionof nitrogen.

224. Lignin or woody fibre exists in almost allplants ; it constitutes the greater part of the stem, bark,and branches of trees, and is present, though in smallerquantity, in the leaves and flowers of trees, shrubs, andsucculent plants. It is the most solid constituent ofplants; giving strength to those parts in which anyquantity of it exists. It may readily be separated fromthe other matters with which it is associated, by bruisingor long-continued boiling : by either of these means the