758 PHYSIOLOGY. 



becomes filtered, as it were, in a great measure, from its impu- 

 rities, before it is brought into contact with them. 



i3esides the use of these cases in growing plants luxuriantly, 

 in those places where, under ordinary circumstances, they would 

 perish, or at all events grow but languidly, they have a still 

 more important application, for they have now been most success- 

 fully employed in transporting plants from one country to another, 

 which under ordinary circumstances would have died in their 

 transit, and whose seeds could not be transported without losing 

 their vitality. The action of the Wardian cases in this mode of 

 transporting plants is twofold : in the first place, the plants are 

 protected from the influence of salt breezes, which are in most 

 instances very injurious to plants; and, secondly, the atmosphere 

 of such cases remains in a quiet state, and by this means the 

 plants are protected from all rapid changes of temperature. 



6. Colour of Leaves. — The green colour of leaves is due to 

 chlorophyll contained in the cells situated beneath the epidermis. 

 Chlorophyll, as already noticed (see page 27), is only formed 

 under the influence of light, and hence the leaves and other parts 

 of plants grown in darkness are blanched or etiolated (page 755). 

 If plants with green leaves be withdrawn from the action of light, 

 and be placed in the dark, these leaves soon fall, and if others 

 are produced, they have a whitish or yellowish colour. Again, 

 if plants, which have been grown in the dark, be removed to the 

 light, the leaves upon them soon lose their whitish hue and 

 become green. The rapidity with which leaves become gre^-n, 

 and the intensity of the colour, will be in proportion to the 

 amount of light to which they have been exposed. 



The diiferent rays of the spectrum have a varying influence 

 in promoting the formation of chlorophyll. Some difference of 

 opinion exists as to those rays which are most active in this 

 respect, but the majority of experimenters agree, that the illumi- 

 nating or yellow rays, namely, those which, as we have already 

 seen (page 752), have the greatest effect in promoting the decom- 

 position of carbonic acid, are those also which are the most 

 active in the prodiiction of chlorophyll. 



M. Yremy, as already noticed (see p. 28), has investigated 

 chlorophyll, and ascertained that it is composed of two colouring 

 principles, — one a yellow, which he has termed phylloxanthin ; 

 and the other a blue, which he has called 'phyllocyanin. Both 

 these principles have been isolated by M. Fremy, who has also 

 endeaA'oured to show, that the yellow colour of etiolated and 

 very young leaves is due to the presence of a body which he has 

 \exraQA. j)hylIox(mtkei7i, and which is coloured blue by the vapour 

 of acids. The same principle results from the decoloration of 

 phyllocyanin ; hence, it would seem, that phj'llocyanin is not an 

 immediate principle, but that it is formed by the alteration of 

 phylloxanthein. The expei'iments of M. Filhol do not, however. 



