EFFECTS OF DEFICIENCY OF LIGHT. 187 



principal ingredients ; and vegetables also require this, for the 

 formation of their solid tissues. The proportion which they take 

 in by the roots is but small ; and what they require in addition, 

 is obtained in this manner from the atmosphere ; as they possess 

 no stomachs, by which it may be introduced in a solid form into 

 the system. It is, then, chiefly of the water absorbed by the 

 roots, and the carbon thus taken in by the leaves, that the 

 elaborated sap or nutritious juice of the plant consists; and the 

 constant liberation of carbonic acid from the general surface, in 

 which the process of respiration consists, appears more necessary 

 to preserve the healthfulness of the general system (by carrying 

 off what is in a state of commencing decay), than to change the 

 character of this juice. 



288. The proportion of carbonic acid, in which a healthy 

 plant will thrive, under the influence of strong sun-light, is not 

 less than 7 or 8 per cent. : but a much smaller proportion than 

 this would soon be fatal to it, if grown in the shade. It is to a 

 peculiar compound formed in the cells of the green surfaces, of 

 which the carbon introduced from the air is an essential ingre- 

 dient, that the colour is due ; and as this fixation can only take 

 place under the influence of sun-light (artificial light, even the 

 most powerful, having no influence on it), plants which grow in 

 dark situations are either in part or entirely destitute of colour. 

 They are then said to be etiolated or blanched ; and the effect is 

 purposely produced in many instances. If the absorption of 

 carbon from the atmosphere is checked, the fluids have a much 

 more watery character, and do not contain their peculiar prin- 

 ciples in nearly so much abundance. Hence many plants, which 

 are rank to the taste and stringy in consistence, when growing 

 in their natural conditions, may be rendered much more palat- 

 able by being blanched, neither the peculiar secretions to which 

 the rankness is due, nor the woody fibres which occasion its 

 toughness, being then formed in the same degree. For this 

 reason it is that Celery, Sea-kale, and many other vegetables 

 are blanched, earth being heaped over their young shoots, so 

 as to keep them from the light. As exhalation will also be 

 checked by the same process, the tissue becomes distended with 



