CONTENTS OF CELLS. 505 



Colours of Flowers. The bright colours of the parts of flowers 

 are produced by substances usually dissolved in the watery cell- 

 sap ; sometimes, however, solid corpuscles or utricular structures 

 are found swimming in coloured cell-sap. 



In young tissues of flowers the colouring-matter may be observed to be 

 formed gradually in the vacuoles of the protoplasm, and, as the cells ex- 

 pand, increasing in quantity until the separate portions coalesce and till 

 the whole cavity of the cell. This is well seen in the coloured hairs of 

 the stamens of Tradescantia. 



The colouring-matters of flowers admit of being grouped in two series, 

 the cyanic series and the xanthic series, with green as an intermediate 

 colour : thus, starting with greenish blue, the cyanic series passes through 

 blue, blue-violet, violet, violet-red to red ; the xanthic series, on the other 

 hand, passes from green to greenish yellow, yellow, orange-yellow, orange, 

 orange-red to red. The cyanic colours are usually in solution ; the xanthic 

 colours are usually solid. It very rarely happens that the colours of the two 

 series are met with in the same flower ; 'hence, though Dahlias and Roses of 

 almost all hues are now to be seen, a true blue tint has never been seen 

 in either ; and there are numerous illustrations of this fact in gardens. The 

 various tints of colour are produced either by the interposition of colour- 

 less cells between those containing coloured juices or by the superposition 

 of cells with different colouring-matter one over the other. Thus an 

 orange tint would arise from the superposition of yellow cells over red, 

 and so forth. White is produced either by a very dilute coloured solution 

 or by the presence of air in comparatively large quantities in the tissues. 

 The velvety appearance of the petals of many flowers is due to the fact 

 that the epidermal cells are raised in the form of small conical elevations 

 like the pile of velvet, and the play of light thereon gives rise to the 

 appearance above mentioned. 



RapMdes. The watery fluids traversing the tissues of growing 

 plants, in consequence of the evaporation from the leaves and the 

 continual absorption by the roots, necessarily contain various in- 

 organic salts dissolved in them. Moreover certain organic acids, 

 such as oxalic, malic, tartaric, &c., are always formed in the pro- 

 cesses of vegetable digestion. All these substances and their com- 

 pounds are, for the most part, dissolved in the cell-sap ; but in 

 most of the higher plants we find, in certain cells of the parenchy- 

 matous tissues, crystals of definite composition, either scattered or 

 collected into groups of definite form. These crystals are called 

 raphides (fig. 555). They are common in certain orders of Flower- 

 ing Plants and Fungi, though others seem destitute of them. 



It is not clear whether the raphides are to be regarded as a secretion 

 or as an excretion that is, as substances useless or noxious to the plant, 

 laid by in an insoluble form. The latter seems more probable, especially 

 as they are usually deposited in tissues of enfeebled vitality. The Poly- 

 gonaceas (for example, the Garden Rhubarb) form abundance of oxalic 

 and other organic acids, and they always contain a quantity of bundles of 

 raphides composed chiefly of calcium oxalate ; in old stems of Cactacea3, 



