230 University of California Puhlications in Agricultural Sciences [Vol. 7 



the inflorescence and furnishes the measurements of the diameters 

 of individual capitula of a single plant. Comparing the individual 

 cymose clusters, the terminal cluster has the largest central flower; 

 closely followed by the next few lower clusters. As the measure- 

 ments are followed farther down, the central capitulum becomes 

 slightly smaller. The lateral capitula are generally smaller thai 

 the central capitulum in each cluster, but at times they may attain 

 to the same size, especially in the uppermost cymes. Very rarely 

 they are larger than the central capitulum of the cyme of which 

 they are laterals. The central capitula of the lower cymes may 

 be larger than the lateral capitula of the upper cymes. In com- 

 paring flower heads as to size, however, the facts that all the capitula 

 do not ripen at the same time and that the age of the plant is a 

 factor causing variation should be kept in mind. Moreover, in this 

 group of measurements, the flowers were pulled off for measuring, 

 and this has a tendency to keep the inflorescence active for a longer 

 time and to maintain the flower size, as has been noted by Goodspeed 

 and Clausen. The facts as to variation of size in the flowers, due to 

 the age and position of the flower, may be summarized by saying 

 that, in plants allowed to flower normally, the terminal flow^er head 

 is usually the largest, closely followed by the second and third flow^er 

 heads, after which the size becomes slightly smaller. The relative 

 size of the flowers on the lower branches is similar, but the terminal 

 flowers on the lower branches are smaller than the terminal flower of 

 the whole plant or than those terminal flowers which arise from 

 branches in the axils of the uppermost leaves. 



Environmental Factors 



Light. — With regard to the effect of light on the flowering of 

 plants, some interesting results have been obtained. Klebs (1918) 

 in his work on Sempervivum divided the process of flower formation 

 into three distinct stages: (1) production of the condition of ripeness 

 to flower, (2) formation of flower primordia, and (3) development 

 of flower clusters and elongation of the axis. He found that light is 

 the dominant factor in determining all three stages. More recently 

 Garner and Allard (1920) have published their opinion that the three 

 primary factors that enter into the action of light upon plants are (1) 

 intensity of the light, (2) quality, that is, the wave length of the 



