CONTENTS OF THE TISSUES. 61 



in the parenchyma, especially in the superficial strata. It consists 

 of minute soft granules, of no particular form, either separate or in 

 clusters, forming grains of considerable size, which lie free in the 

 cells, or loosely adhere to their sides. They often adhere to the 

 surface of starch-grains. Indeed, Mr. Henfrey plausibly considers 

 chlorophyll to arise from altered starch (with the evolution of oxy- 

 gen) ; which is the more likely, as it is said to appear in the cells 

 later than starch.* It belongs to the class of waxy bodies ; and is 

 soluble in alcohol or ether, but not in water. Chlorophyll under- 

 goes certain changes, in autumn foliage especially, by which it 

 turns to red or yellow. CHROMULE is a name applied to coloring 

 matters not green, and mostly in a liquid form, as in the cells of 

 petals, giving to them their peculiar tints. These coloring matters 

 are probably a mixture of very various products. 



88. Alkaloids (such as Morphine, Strychnine, and Quinine) are 

 quaternary products of plants, principally formed in the cells or 

 interspaces of the bark. Unlike the proteine compounds (27, 79, 

 gluten, fibrine, &c.), they appear to bear no part in vegetation, but 

 to be completed results of vegetation, and therefore of excretory 

 nature. In these substances reside the most energetic properties 

 of the vegetable, considered as to its action on the animal econo- 

 my, the most powerful medicines, and the most virulent poisons. 

 That they are of the nature of excretions may be inferred from 

 the fact, that a plant may be poisoned by its own products. 



89. Tannin OF Tannic Acid, which most abounds in older bark, is 

 probably a product of the oxidation or commencing decomposition 

 of the tissues. So, also, Humus, Humic Acid, Ulmin, Ulmic Acid, 

 and the numerous related substances distinguished by the chemists, 

 are products of further decomposition of vegetable tissue, and not 

 products of vegetation. 



90. Vegetable Acids, Tartaric, Citric, and Malic acids are the 

 principal kinds, which occur in leaves and those succulent stems 

 which have a sour juice, and in all acidulated fruits. They are 

 ternary products, with an excess of oxygen. Oxalic Acid, which 

 is an almost universal vegetable product, is a binary body, differ- 

 ing from carbonic acid in ultimate composition only in having a 



* In that case, the nitrogen obtained in Mulder's incomplete analysis (which 

 gave C 18 , H 18 , N 2 , O 8 , with some nitrogenous matter not determined) must 

 belong to the mucous matter, or protoplasm, which invests the green granules. 



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