BOTANY. Ill 



The principal results are summarized as follows: 



The greatest expanse of leaf blade was in those plants under blue 

 light, the least under the green light, with the red intermediate. Those 

 under the blue light approached most nearly to the total illumination 

 and the green to darkness. The effect on the petioles varied but in 

 two ways. In one series the green rays produced the longest petioles, 

 with the shortest under the blue, and intermediate under red illumina- 

 tion. In the other the order was reversed. With those plants having 

 a rosette of root leaves the longest leaves grew under the green screen 

 and the shortest under the blue, with red intermediate. At the same 

 time the longest leaves were not always the largest. All the different 

 colored lights were less favorable to the development of the tissues of 

 the leaves than the white light. 



The amount and development of the palisade tissue, parenchyma, 

 and air spaces were least under green light, greater under red, and 

 most highly developed under blue light. The chloroleucites, so far as 

 number, size, and disposition were concerned, were similar in develop- 

 ment to the assimilative tissues. Under the green light they were 

 small, fewer, of indefinite shape, and distributed without any order in 

 the cell and did not contain as much chlorophyll as either under the 

 red or blue. The number of stomata per unit of leaf surface was 

 greatest under the green light, less under the red, and least under the 

 blue. The development of wood, liber, and cambium of the veins, as 

 well as the lignification of the cell walls of the leaves, was the same as 

 in the stems and roots. 



The studies of roots showed that for plants whose roots are retarded 

 by white light, blue light retarded them still more, while under green 

 light they attained greater length, although the maximum development 

 was in darkness. When roots developed better in light than in dark- 

 ness they showed increased growth under blue screens and made little 

 increase under green. When growing equally as well in light and 

 darkness the different colored lights made no appreciable difference. 

 The maximum diameter of the central cylinder and thickness of cortex 

 of roots was shown in the plants grown under red and Uue light, with 

 green as the minimum. Under the green light the primary wood 

 presented few vessels and the differentiation of secondary tissues was 

 less advanced. The same was true for the lignification of cell walls 

 and supporting tissues. Under the green light the structure of roots 

 approached those grown in darkness, while those under blue light 

 were more nearly like those grown under white light. 



The investigations showed that the green light gave the greatest 



growth of stem, followed by red and blue when the experiment was 



not continued beyond the consumption of the reserve materials of the 



plant. When conducted longer the plants under green light perished. 



4740— No. 2- 2 



