without fresh Supplies of Water and Air. 493 



and in which changes of temperature proceed so slowly, tropical 

 plants may, perhaps, bear, without injury, degrees of cold which 

 would prove fatal if occasioned by the frigorific operation of 

 free air in constant and rapid motion. 



7. Condition of Plants, with regard to Light, in close Cases and in 

 the free Atmosphere. 



Of the great importance of light to vegetation Mr. Ward is 

 fully sensible. " The success of his plan," he says, " will be 

 in proportion to the admission of light to all parts of the grow- 

 ing plants. In every place," he adds, " where there is light, 

 even in the centre of the most crowded and smoky cities, 

 plants of almost every family may be grown " by this method. 

 Seeds, it is well known, germinate best when buried in the 

 soil, and entirely secluded from light; but when the young 

 germ pushes into day, if light be still excluded, by inverting 

 over it an opaque vessel, the plant shoots up into a long and 

 feeble stem, is of a pale or whitish yellow colour, and possesses 

 little odour or savour. On the contrary, if the vessel be trans- 

 parent, so as to transmit light, the growth is more vigorous, the 

 young plant puts forth buds from its stem, and soon exhibits its 

 characteristic form and colour. 



These differences in form and colour, according as plants are 

 grown in light or in darkness, were early noticed by Ray, and 

 afterwards by M. Bonnet, in his Recherches sur V Usage des 

 Feuilles, p. 210. In the year 1771, Dr. Irvine described still 

 more minutely the influence which light exerts on vegetation. 

 Plants, says he, though furnished with water, heat, and air, 

 grow imperfectly, if placed in a dark box, and never contain 

 any thing but a watery juice; hence the rays of light are in 

 some way necessary to the perfect growth of vegetables ; since, 

 when deprived of this influence, they all agree in the nature 

 and qualities of the juices they contain ; nor have they that 

 variety in colour and flavour which they had before. The most 

 pungent vegetables become insipid, the highest-scented inodo- 

 rous, and the most variegated in colour of a uniform white- 

 ness, when secluded from light. Vegetables, too, which grow in 

 a natural situation, readily burn when dry ; but a vegetable 

 reared in a dark box contains nothing inflammable. {Essays on 

 Chemical Subjects, p. 430.) In regard to colour and smell, 

 similar observations were made by Professor Robison on tansy 

 (Tanacetum vulgare) and other plants, which, when grown in 

 darkness, were white, and afforded no aromatic smell ; but, when 

 brought into daylight, the white foliage died down, and the 

 stocks then produced the proper plants in their usual dress, and 

 having all their distinguishing smells. {Black 's Chemical Lectures, 

 by Robison, vol. i. p. 532.) 



