274 



THE AGRICULTUKAL NEWS. 



September 



1911. 



reflected from surrounding surfaces, and that reflected 

 from the ground. The firstof these influences the arrange- 

 ment of the leaves on the branches, their position, and the 

 manner in which the branches are developed. The side 

 light stimulates the growth of the buds that are borne 

 on lateral branches; its effect is ofoen seen in the <level- 

 opment of the branches on the side of an opening in 

 a wood or forest. All these must be considered in rela- 

 tion to the illumination of any individual plant, and 

 attention nitist be given to the fact, as well, that trees 

 rarely receive the total daylight, for this is lessened in 

 amount by the shading caused by the foliage of the 

 plant itself and by neighbouring trees. 



In dealing with the behaviour of diflerent kinds of 

 trees, under varying conditions, regard is had to their 

 tolerance of shade, or as it is often termed meiely, their 

 tolerance. Trees which endure shade well are said to be 

 tolerant, while those needing full light are termed in- 

 tolerant. Most trees, even in the tropic-^, will thrive in 

 full light, particularly if they have grown under this 

 condition from the time of planting out. Different 

 plants, in the open, do not however employ the same 

 amount of light: they are enabled to use the quantity 

 that suits them best by placing their leaves in such 

 a position, relative to the source of light, as to regulate 

 the amount falling upon them; or the density of their 

 crowns may be increased so as to produce a deeper 

 shade for the lower and inner branches: or ihe structure 

 of the leaves may be altered, even, in order to lessen 

 their power of assimilation. Tolerance, nevertheless, is 

 not a flxed quality, in regard to any one kind of tree, 

 since plants may change some of their characters in 

 relation to their environment; while the latter cannot 

 have any effect upon the amount of light which is 

 required inherently. 



After treating of these matters, the Bulletin men- 

 tioned makes short reference to the work of Lubimenko 

 and of (irafe, which has shown that the amount of 

 assimilation carried on by the leaf is influenced directly 

 by its anatomical structure and by the properti'^s of the 

 specialized portions of the protoplasm (chloroplasts) 

 Containing chlorophyll, in their cells. This work has 

 also led to other interesting conclusions, aiuong which 

 are the fact that the regulation of assimilation takes 

 place, in medium diffused light, mainly through the 

 influence of the first mentioned cause, while the proper- 

 tics of the chloroplasts have their greatest effect in light 

 at great and small intensities; and that the latter cause 

 determines the stage at which the most useful amount 

 of assimilation takes place. 



Considei;'ation is given to experiments that have 

 been carried out for the purpose of ascertaining the 

 most usual cause of the death, or poor development, of 

 undergrowth in forests and woods. These have shown 

 that the state is not caused by insufiicient light alone, 

 but by the competition of the roots of the large trees: 

 the effect of the latter is to lower the water content of 

 the soil to such a degree that the smaller plants are 

 prevented from ri<jurishing. This serves to exphiin the 

 effects of thinning, in closely growing plantations, 

 whcreb}- the lessened competition for water among the 

 roots permits of increased growth on the part of the 

 plants that remain. 



As regards the intensity <jf light needed at differ- 

 ent stages of growth, it is a general fnoo that most 

 trees exhibit tolerance when they are voung-, and 

 that as the)' become ol<ler they require more light, 

 while the differences between various species in this 

 respect become more marked. The demand for light 

 also changes with the season : more is wanted for 

 flowering and fruiting than at other times. These 

 matters are such as might be expected, when it is 

 considered that an increase in the rate of formation of 

 tissue entails an enhanced rate of assimilation, and thus 

 a greater degree of illumination. 



There are several methods of comparing the 

 amount of tolerance exhibited by trees of different 

 species. Among these are observation of the density 

 of the crowns, of the rate of natural pruning and thin- 

 ning under similar conditions, of the rapidity of growth, 

 of the ability of seedlings to flourish in the shade, and 

 the determination of the ratio between the height of 

 a tree and its diameter. One of the most useful of these 

 methods is the second, namely observation of the 

 rapidity with which self-pruning of the lower branches 

 takes place. As regards the last, the ratio is usually 

 teimed the relative height of the tree: its usefulness 

 depends on the fact that the greater the tolei-ance of a 

 tree, tlie less is it likely to increase in height in a 

 manner disproportionate to the rate of growth of its 

 diameter. Thus in the case of trees that have died 

 under shade, the relative height is represented by a 

 comparatively large number. 



It will be well to mention the effects of shading 

 on fhe leaves of plants, as they are given in the work 

 under consideration. In the first place, it causes well- 

 marked changes in the anatomical structure; secondly, 

 as is well known, it prevents the leaves from assuming 

 their natural green colour. Further, shaded leaves 

 show a smaller tendency to the production of a crumpled 



