LECT. XI. ACTION OF LIGHT ON VEGETABLES. 217 



The influence of the luminous rays on germination has 

 been of late spoken of. Some observers have asserted, that 

 the violet, or chemical rays, promote it ; whilst others main- 

 tain an opposite opinion. This contradiction is a fresh proof 

 of the necessity of having recourse to more exact experi- 

 ments. It is not difficult to discover the source of the dif- 

 ferent results obtained by experimentalists, since they have 

 not employed the simple rays of the solar spectrum ; but the 

 coloured rays obtained by the passage of solar light through 

 glasses of different colours. Now, in general, a solar ray 

 which traverses coloured glass, is not deprived of all the 

 rays which possess colours different to that presented by 

 itself. 



A curious effect of light upon vegetables is seen in the 

 tendency which certain roots have to avoid it ; and, on the 

 contrary, which others evince to seek it. The roots of many 

 plants belonging to the family Crucifera, do the former ; 

 those of Mlium cepa, the latter. According to Dutrochet, 

 the intimate structure of the cortex of roots, is different ac- 

 cording as they seek or avoid the light ; and from this dif- 

 ference results the tendency which they evince to direct 

 themselves one way or the other. Generally, in the barks 

 of young plants, the utricles are largest in the median 

 layers of its thickness, and decrease in size as they approach 

 both towards the interior and the surface ; but in some 

 cases this decrease is less towards the outer layers ; while 

 in others, it is less towards the inner ones. Under the in- 

 fluence of solar light and heat, the plant transpires and the 

 utricles yield up the water they contain. It follows, there- 

 fore, that the roots direct themselves towards the light, 

 when the structure of the internal layer of the bark is 

 denser than that of the external ; and the reverse effect 

 takes place when the external layers possess a greater 

 density. 



