UPON PLANT-HOUSES, 89 



The most impenetrable of all wonders are the three-twins : 

 light, heat, and shadow. They appear ever combined together, 

 and as regards the two first, they grow into each other in such 

 a wise, that no mortal has ever pointed out then" line of demarca- 

 tion ; while the third brother, shadow, is an unsubstantial 

 negative, likewise intimately united with the two others. Unseen 

 it traverses the eye of the smallest needle, and the widest 

 church gate ; only when its brothers are obstructed in their 

 course does it make itself perceptible. The vegetable world 

 is dependent on light and heat, as well as their negative, shade. 

 Leaving out of consideration the class of countless Diatomese 

 which have been given to science, if not to the animal kingdom, 

 by the I'esearches of Ehrenberg ; and the fungi of dark and damp 

 recesses, the world of green plants is so powerfully influenced by 

 light and shade, that we must bring certain practical results, 

 connected with them, to bear upon the construction of our glass- 

 houses. Gustav Heyer has recently published a graduated list of 

 timber trees'-' which endure shade, or require light. 



The author points out that the relation of these important 

 timber trees towai'ds light and shade, is indicated by their 

 dense or light foliage (in forests), the extent of time in which 

 oppressed branches and stems will keep alive, and the ability of 

 the young plants to thrive under the old stems. But this 

 dependence on light and shade becomes modified by interfering 

 circumstances. Thus, on rich and fertile, or moist lands, plants 

 which would otherwise require light, will thrive in shade. In the 

 rich loam of the mild Wetterau, potatoes and grain thrive quite 

 freely under the shade of fruit-trees ; while a little further north- 

 ward, in the less fertile soils of Giessen and Marburg, the area 

 under trees remains naked. Great difference occurs also in hilly 

 situations, where frequent fogs and rain prevail. We must bear 

 in mind, further, that northern declivities of mountains have often 

 a thicker layer of mould than southern ones, in which it becomes 

 more rapidly decomposed and is dispersed, as may be observed in 

 primaeval forests of tropical countries. All these matters should 

 assist us in our work, as far as the scanty information we possess 

 concerning the specific relation of tropical plants towards light 

 and shade, &c., will enable us. But, in fact, we are unable satisr 

 factorily to limit the boundaries between the luminous and 



* Das Vei-halten, &c. (" Relation of Forest-trees to Light and Siiade.") 

 -Erlaugen, 1852. 8vo, p. 3. 



