3 02 PLANT-BREEDING 



but at present it is known that the faculty of producing them 

 must be present beforehand, and that the food-supply can 

 only stimulate the frequency of occurrence and the degree 

 of development. 



We may now proceed one step farther and inquire into 

 the time at which the external influences can affect the size 

 and the form of the organs. It is evident that these agencies 

 must be limited to the period of youth, because as soon as 

 the growth ceases, no further change is possible. Here, 

 however, the form and the size must be considered separately, 

 the form being definitely fixed long before the size of the 

 organ is determined. We thus come to this conception 

 in regard to periods of sensibility, that the one for the form 

 ceases earlier than that of the ultimate increase. 



The pistillodous opium-poppy may once more be chosen 

 as an example. If we examine a young plant six or seven 

 weeks old we find a short stem bearing a group of leaves 

 which surpass it in size many times. The young bud of 

 the flower is attached at the head of the stem. Its calyx 

 covers the innermost parts, which are still very small. The 

 central capsule hardly begins to be differentiated and the 

 whorl of stamens is seen as a smooth wall of soft tissue 

 around it. At this period the normal and the pistillodous 

 stamens are still awaiting the first manifestation of their 

 form. Shortly afterward they may be seen as small protuber- 

 ances, rounded and without distinct form, but gradually 

 increasing and assuming more definite types. At this time 

 there is still no visible distinction between the normal and 

 the pistillodous stamens, and it is not possible to decide 

 whether the bud would grow into a rich or into a poor 

 representative of the variety. A few days afterwards, 

 however, the decision becomes apparent, and the period 

 of sensibility closes. 



Exact limits for this period of sensibility to the external 



