26 EFFECTS OF AETIFICIAL SHADING ON PLANT GEOWTH. 



were immediately retarded in growth by the application of shade. 

 This was not true for the ordinary mesophytic plants but for such 

 plants as Salsola tragus, Amarantltus blitoides, and Atriplex canescens, 

 which are all xerophytic, sun-loving plants. 



Most of the species used showed increased growth in shade. 

 Combes (1910) concludes from his experiments that seedlings are 

 more tolerant of shade than are adult plants. From the data 

 obtained both by Combes and by the writer it appears likely that 

 tolerance of shade is largely determined by the ability of the plant 

 to manufacture food. If this be the case, seedlings, because of the 

 food stored hi the seed, would be expected to show much greater 

 tolerance than adult plants, which are entirely dependent upon 

 photosynthesis to support further growth. 



The results of the experiments described in this paper make it 

 seem probable that only during the periods of most intense iHumina- 

 tion was a light intensity of n/15 sufficient to permit the elaboration 

 of starches in any of the plants grown. (PL VI.) Moreover, none 

 of the native plants which grew as weeds at the edge of the beds under 

 the different shades were able to manufacture starch when the light 

 intensity was below n/15. 



It was also evident that where the illumination exceeded n/15 

 cotton, lettuce, potato, and radish were able to elaborate organic 

 matter as readily as under full light. 



Careful observation of the growth of these plants for the period 

 during which the experiment was continued showed in practically 

 all cases an increased growth where there was a slight reduction of 

 the light intensity. A decided increase in growth could be noted in 

 passing from normal to one-half normal light in all cases with the 

 exception of corn. 



In general no reduction hi growth occurred when light was reduced 

 to n/5, and usually none took place at n/7. But when the light was 

 reduced to n/15 a very marked falling off hi growth occurred (PL VI) 

 and hi most species the plants disappeared entirely under the shade, 

 giving an illumination of n/93. Mustard made its best growth in 

 light of an intensity between n/5 and n/2, and radish between n/7 

 and n/5, with a decided reduction of growth hi full light. Lettuce 

 grew best where the light was between n/2 and n/7, the plants under 

 full light behig smaller by comparison. Cotton produced the largest 

 and most vigorous plants between the shades corresponding to n/5 

 and n/2 light intensity, the plants in full light having been much 

 smaller. In n/15 growth was almost checked, while in n/93 the plants 

 continued alive but were unable to elaborate any food material. 

 Full light seemed unfavorable to maximum growth in the potato 

 but not in corn. While the potato and corn were able to maintain 



279 



