444 THE BIOLOGY OF FLOWERING PLANTS 



an indirect effect through photosynthesis. The supply of 

 salts and of water also affects various metabolic processes. 



A. M. Smith (1907), in a study of the growth-rates of 

 plants in Ceylon, has shown that Blackman's conception of 

 Hmiting factors is applicable to the regulation of growth. 

 Now one factor, now another, is limiting. The growth 

 of Capparis was limited by water supply through the day 

 and by temperature at night ; that of Vitis by water supply 

 in July and temperature in January. The same relation 

 has been formulated as the " law of the minimum " by 

 Meyer (1895) for growth factors such as the supply of the 

 various mineral salts. It should be noted that, while the 

 direct effect of such factors as light, temperature, and 

 water supply is immediate, their indirect effect, through 

 assimilation, as well as the effect of the mineral supply, tends 

 to be felt gradually and cumulatively ; it is diffused and may 

 be delayed. Balls (191 8), in his studies on the cotton in 

 Egypt, found that the action of a particular factor might 

 become visible, e.g. on the rate of flower production, weeks 

 after its incidence, and this he calls the " principle of 

 predetermination." 



§ 9. Development — The Vegetative Phase 



In the course of its development the plant passes through 

 a series of stages the most prominent of which are the 

 vegetative and the reproductive. We have mentioned a 

 case of a plant {Cardamine chenopodiifolia) which produces 

 flowers at a very early point, when only two leaves have 

 appeared ; but ordinarily a more or less prolonged vege- 

 tative phase precedes reproduction. The vegetative phase 

 does not, however, consist in the production of a series of 

 uniform organs. This may be seen by the examination of 

 almost any young plant. If we leave the cotyledons out 

 of account, the first-formed foliage leaves are almost invari- 

 ably small, and they tend to be smooth in outline ; if the 

 adult leaves are lobed or cut the juvenile ones may be almost 



