1Q2 PHYSIOLOGICAL BOTANY. PART II. 



For the present then, we may consider this substance 

 as most probably the material which is primarily pre- 

 pared for the nourishment of all parts of the vegetable 

 structure, and which is afterwards further modified by 

 the different vesicles and glands distributed through the 

 system, according as the nature of different parts may 

 require. 



(178.) Etiolation. When any part of a plant capa- 

 ble of decomposing carbonic acid is entirely excluded 

 from the light, it remains white. This "etiolation," 

 as botanists term the phenomenon, consists in a combin- 

 ation of an excess of water with the vegetable matter 

 previously prepared ; so that the quantity of carbon 

 already fixed becomes as it were diluted, and diffused 

 over a wider space. If the etiolated parts are exposed 

 to the light, the green colour makes its appearance in 

 less than eight-and-forty hours, and the plant gradually 

 assumes a natural and healthy character. The parts 

 which have once become green are incapable of being 

 completely etiolated afterwards. Among the various 

 vegetable matters used by man as food, those which 

 are the least sapid are among the most alimentary ; 

 whilst the more highly flavoured are generally more or 

 less deleterious, and some of them extremely poisonous. 

 In order to obtain a food which shall be both whole- 

 some and grateful, the horticulturist contrives by vary- 

 ing his mode of culture to moderate the proportion in 

 which the deleterious ingredients are naturally secreted, 

 and thus renders them harmless. The most common 

 mode of producing this effect is by removing the sti- 

 mulus of light from such parts as are intended to be 

 eaten ; this both diminishes the activity of the organs 

 employed in secreting the deleterious matters, and at the 

 same time causes them to absorb a superabundant supply 

 of moisture. In this way the blanched stems of celery, 

 which in its natural state is a poisonous plant, become a 

 grateful food. The leaves of the endive, and many 

 others which would be far too bitter or tough in their 



