270 PLANT PHYSIOLOGY 



sexual reproduction can, therefore, be ascertained by study- 

 ing those forces which influence the conspicuous parts, the 

 flowers. A force which must attain a certain strength before 

 it will stimulate the plant to blossom, and without which 

 the plant blossoms but imperfectly if at all, is one which 

 controls sexual reproduction. Without this force the plant 

 may reproduce itself by non-sexual means, by vegetative 

 processes such as the formation of runners, suckers, etc. 

 Under the influence of this force it will multiply only by 

 sexual means. In such a case, the one force would control 

 reproduction, both sexual and non-sexual. The effect of the 

 force as a controlling agent is due to its own direct influ- 

 ence upon the plant and also to the irritable response of the 

 plant to its influence, the response setting in motion other 

 forces in its own body. The force from without is but the 

 initial energy which releases other quantities of energy, thus 

 setting in operation other processes than those which it 

 can directly affect. 



Of the many plants studied by Vochting, Mimulus Ti- 

 lingi serves best to illustrate the facts which he has brought 

 out. Figures 22 and 23 in the accompanying illustration 

 indicate the normal habit of potted plants of this species, 

 respectively a blooming and vigorous plant from a last 

 year's cutting, and a much younger one not yet ready 

 to bloom. The vegetative functions, as weU as the vegeta- 

 tive mode of reproduction, of the plant, are carried out by 

 the usually more or less creeping leafy branches originating 

 on the stem just above the surface of the soil. When the 

 vegetating period draws toward the close, the new in- 

 ternodes of these branches are successively shorter, the tips 

 of the branches forming finally rosette-like bunches of leaves 

 on the surface of the soil. Such a bunch cut off and tran.s- 

 planted will presently give rise to the flowering erect 

 branch shown in figure 22. The rosettes will form erect 

 branches, however, only when warm enough. The upper 

 part of an erect branch bears flowers. Just below these are 

 a few pairs of leaves in the axils of which short vegetative 

 branches begin to form. The flowering part of the branch 

 may bear lateral flowering branchlets if the plant is vig- 



