UPON PLANT-HOUSES. 87 



thus the commenchig forest thins itself, according as the locality 

 is favourable or otherwise. The denser the crowd is, the less are 

 the young trees able to send forth branches, and the more are 

 these shortened and impoverished ; while, at the same time, the 

 stems themselves push on rapidly in perpendicular height, in 

 order to lift their crown towards the light. Gradually fertility 

 comes on, the forest extends itself centrifugally in all directions, 

 most of the seedlings under the shade perishing off in their 

 infancy. The open plain is now converted into a green and 

 shady forest ; the underwood is gradually suppressed by the 

 powerful growth of the trees, giving way to a vegetation of shade- 

 plants and of mosses. But now the parent tree shares the lot of 

 part of its own progeny ; these deprive it of its means of existence, 

 the soil furnishes insufficient food, the air supplies inadequate 

 light, heat, and moisture, and gradually it dies down to the root. 

 Such is the universal course of nature ; and, modified according 

 to local diversity, it prevails all over the vegetable world. The 

 sagacious forester knows how to turn this to advantage, by 

 thinning and other processes. He notices that where a vacant 

 space occurs, the nearest trees extend their branches most actively 

 in that direction ; and that the trees on the skirts of the forest 

 produce the strongest branches outwardly; in short, that the 

 trees, as well as their boughs, entirely obey the light and its 

 concomitant heat in their development — that this is greatest on 

 the open, least on the shady side. But he perceives at the same 

 time, that reflected heat and its radiation, as well as the quality 

 of the soil and its moisture, essentially qualify these effects ; that 

 trees differ much in their predilection as regards light and 

 shade ; some demanding the former, some requiring the latter, 

 while others will barely, or not at all, endure shade ; finally, that 

 all these combined phenomena vary according to the different 

 periods of life in individuals. 



In a narrow valley, extending south and north, all other con- 

 ditions being equal, the western aspect will be most luxuriant, 

 because the solar influence is greater and lasts longest on that 

 side, producing greater radiation. If vertical rocks are present, 

 especially of white colour, there will be an active reflection of 

 light, illuminating as well as heating. In case the direction of 

 the valley is east and -west, the vegetation will be strongest 

 towards the south from a similar cause. A tree on a northern 

 declivity of a hill will send forth more slender branches to the 

 north, and stouter and more leafy to the south, and vice versa. 



