744 PHYSIOLOGY. 



cover, and in so doing, becomes filtered, as it were, in a great 

 measure, from its impurities, before it is brought into contact 

 with them. 



Besides 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 

 successfully 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 

 either without losing their vitality. 



The action of the Wardian cases in this mode of transport- 

 ing plants is twofold ; thus, 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 20), is only formed 

 under the influence of light, and hence the leaves of plants 

 grown in darkness are blanched or etiolated (page 741). 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 plants with green leaves be withdrawn from the action of light, 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 green, 

 and the intensity of the coloui*, will be in proportion to the 

 amount of light to which they have been exposed. 



The different 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 738), have the greatest effect in promoting the decora- 

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

 active in the production of chlorophyll. 



M. Fremy has recently made some important investigations 

 upon chlorophyll. He has ascertained that it is composed of two 

 colouring principles, — one a yellow, which he lias termed phyU 

 loxanthine ; and the other a blue, which he has called phyllo- 

 cyanine. Both these principles have been isolated by M. Freray. 

 M. Fremy has also endeavoured to show, that the yellow colour 

 of etiolated and very young leaves, is due to the presence of a 

 body, which he has termed phylloxanthcine, and which is 

 coloured blue by the vapour of acids. The same principle 

 results from the decoloration of phyllocyanine ; hence, it would 



