i 909 ] HILL— POLLINATION IN LIN ARIA 463 





IV. LIGHT AND 



The behavior of Linaria canadensis led to the conclusion that 

 the gradual diminution in size of flowers was connected with the 

 increase of heat, and perhaps of the light, to which they are exposed. 

 Taking the larger plants as typical examples, the two features are 

 in inverse proportion. This might be taken as a coincidence, but 

 it seemed to be explained better as a coordination, and more in har- 

 mony with observations and experiments by others. In 1874 Bouch£ 

 called attention to his observations that the diminution in the size 

 of flowers and the production of cleistogamy depend in some plants 

 on the decrease or increase of heat, in others on the decrease or increase 

 of the length of the day. In the behavior of some, of which Vinca 

 rosea L. is an example, the light acted favorably, the largest flowers 

 being formed during the longest days, the smallest during the season 

 of the shortest days. This seemed to depend on the light, since 

 with a higher temperature after the longest days had gone by the 

 decrease went on. In other cases cited by him, the decrease in size 

 and production of cleistogamy are coordinated with the increase of 

 heat and light, as if these acted unfavorably. As examples of this 

 are the malvaceous plants, Pavonia hastata Spr. and P. praemorsa 

 Willd. They begin to bloom at the end of May and show the phe- 

 nomena of diminution and cleistogamy until the autumnal equinox, 

 after which the flowers gradually increase in size till the beginning 

 of winter or close of their floral season.' 8 The case of the pavonias 

 more closely accords with that of L. canadensis, as far as the floral sea- 

 son of the two coincide. But since, according to Bouch£, the effects 

 are not uniform, and may even lead to opposite results with different 

 plants, there must be something in the plants themselves which 



the different response, or other environmental conditions 

 must be taken into account. In the case of Linaria, I had associated 

 it chiefly with the increase of heat which ordinarily occurs in summer, 

 and the consequent diminution or more rapid removal of the moisture 

 from the soil of such localities as the plants frequent. The equilib- 

 rium between absorption, either from the air or ground, and transpira- 

 tion is disturbed. The smaller or cleistogamic flower, requiring less 

 food, permits a husbanding of resources for the production of seed. 



<3 BouCh£, Gesells. naturf. Freunde 90, 91. 1874. 



causes 



