limbers. 



120 



[February, 1912, 



erent plants, in tithe open, do not, how- 

 ever, 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 den- 

 sity of their crowns may be increased so 

 as to produce a deeper shade for the 

 lower and inner branches ; or the struc- 

 ture of the leaves may be altered, even, 

 in order to lessen their power of assimi- 

 lation. Tolerance, nevertheless, is not a 

 fixed quality, in regard to any one kind 

 of tree, since plants may change some of 

 their characters in relation to their envi- 

 ronment; while the latter cannot have any 

 effect upon the amount of light which 

 is required inherently. 



After treating of these matters, the 

 Bulletin mentioned makes short refer- 

 ence to the work of Lubimenko and 

 of Grafe, which has shown that the 

 amount of assimilation carried on by 

 the leaf is influenced directly by its 

 anatomical structure and by the pro- 

 perties of the specialized portions of 

 the protoplasm (chloroplasts) contain- 

 ing chlorophyll, in their cells. This 

 work has also led to ether interest- 

 ing conclusions, among which is the 

 fact that the regulation of assimilation 

 takes place, in medium diffused light, 

 mainly through the influence of the first- 

 mentioned course, while the properties 

 of the chloroplasts have their greatest 

 effect in light at great and small inten- 

 sities ; and that the latter cause deter- 

 mines the stage at which the most use- 

 ful amount of assimilation takes place. 



Consideration is given to experiments 

 that have been carried out for the pur- 

 pose of ascertaining the most useful 

 cause of the death, or poor development, 

 of undergrowth in forests and woods. 

 These have shown that the state is not 

 caused by insufficient 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 flourishing. This serves 

 to explain the effects of thinning in 

 closely growing plantations, whereby 

 the lessened competition for water 

 among the roots permits of increased 

 growth on the part of the plants that 

 remain. 



As regards the intensity of light need- 

 ed at different stages of growth, it is 

 a general fact that most trees exhibit 

 tolerence when they are young, and that 

 as they become older 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 con- 

 sidered 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 compar- 

 ing 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 prun- 

 in gand thinning under similar conditions, 

 of the rapidity of the 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 termed the relative 

 height of the tree ; its usefulness depends 

 on the fact that the greater the tolerance 

 of a tree, the less is it likely to increase 

 in height in a manner disproportionate 

 to che rate of growth of its diameter. 

 Thus in the case of trees that have died 

 under shade, the relative height is repre- 

 sented by a comparatively large number. 



It will be well to mention the effects 

 of* shading on the leaves of plants, as 

 they are given in the work under con- 

 sideration. 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 surface 

 and of hairs, than those in bright light, 

 while the veins of the former are less 

 strongly developed. Lastly, shaded 

 leaves are usually thin and limp, and 

 do not possess the dense structure of 

 those that have received a normal supply 

 of light. 



Enough has been said to show that 

 the supply of light to plants is of much 

 importance in relation to their life-his- 

 tory, and that this is the case to such an 

 extent as to influence them in the direc- 

 tion of making changes in their structure 

 in order that they may adapt themselves 

 to the particular circumstances of the 

 illumination in which they have to grow. 

 This importance is being recognized for 

 forest plants, and there is no reason 

 why it should receive less attention in 

 relation to plants, such as cacao and 

 limes, raised in orchard cultivation, and 

 to the proper provision of shade trees 

 for plants growing under artificial 

 conditions. 



