657 



furthers the formation of the leaf shoots and can, in fact, cause the trans- 

 formation of an inflorescence bud into a climbing branch. Direct sunshine 

 has an exactly opposite effect. 



Green's experiments^ are very important for pathology and especially 

 for the point of view which we would represent, that a whole series of 

 diseases is caused by a change in enzymatic functions. He confirms the 

 observations of Brown and Morris that the supply of diastase in the foliage 

 is diminished after a period of bright illumination. The ultra violet and 

 adjoining visible rays are especially important in producing such an enzy- 

 matic decrease. Such an enzymatic destruction by light may be compared 

 with the well-known killing of bacteria by light. 



Shading. 



In agriculture, the injuries produced b}^ direct etiolation are much less 

 frequent and, on this account, less significant than the lower grade of occur- 

 rences which arise from an insufficient supply of light; i. e., too strong 

 shading, and make themselves felt in the decreased production of useful 

 substances. Stebler and Volkart- have made measurements of the removal 

 of light caused by different trees. With a clouded sky, they found a 

 decrease of light from the pine of 50 per cent. ; from the birch, 56 per cent. ; 

 from the cherry, 78 per cent. ; from the oak, pear and apple, 82 per cent. ; 

 and from the beech, 95 per cent. 



Since each plant has its definite need of light, cases also occur in which 

 cultivation gives an excess of light, while the natural habitat would furnish 

 the plant with only a subdued amount. This is found in many of our hop 

 fields and in strawberry culture^. In such cases, shade causes an increased 

 production but, in the majority, reduces the amount of dry substance and 

 weakens the color of the foliage and blossoms. The question of shading 

 may be of especial importance for our colonial plants. In Java, as well as 

 in our East Africa colonies, coffee plantations suffer very frequently and 

 Zimmerman* ascribes this to a lack of shade trees which would prevent 

 over-production by the coffee trees ; for example, in Usambara, this has 

 already caused great injury. It is probable that the consequent lessened 

 strength of illumination, besides the protection from the wind and decrease 

 of temperature, especially favors the thriving of coft"ee. 



The decreased harvest from plants which need light, due to the influ- 

 ence of the shade of trees, arises not only from the limited amount of light 

 but also from the lesser warming of the soil. E. v. Oven's experiments^ 



1 Green, .J. Reynolds. On the action of light on diastase. Phil. Trans, of the 

 R. Soc. of I^ondon. Ser. B. Vol. 188; cit. Bot. Jahresber. 1897. I, p. 89. 



- Stebler, F. G., u. Volkart, A., Der Einfluss der Beschattung- auf den Rasen. 

 Landwirtsch. Jahrtaucher. d. Schweiz. Bern 1904; cit. Bot. Centralbl. 1908, Vol. 

 101, p. 60. 



3 Taylor, O. M., and Clark, V. A., An experiment in shading- strawberries. New 

 York Agrie. Exp. Sta. Geneva Bull. 246. 1904. 



4 Zimmerman, A., Einige Bemerkungen zu dem Aufsatze von Fr. Wohltmann, 

 usw. Berichte iiber Land- u. Forstwirtschaft in Deutsch-Ostafrika. Vol. I, Part 5, 

 1903. 



5 V. Oven, tJber den Einfluss des Baumschattens auf den Ertrag- der KartofCel- 

 pflanze, Naturw. Zeitschr. f. Land- u Forstwirtschaft. 1904, p. 469. 



