THE PRIMORDIA 57 



Fagus silvatica) in ordinary years (in Flanders) from about the 

 end of April to about the middle of May. The period of cell- 

 division is included within the bud period ; therefore the 

 plasticity of the primordium number of cells depends on external 

 causes (temperature, etc.) which are acting in July, August, 

 September, but this primordium is, of course, not affected by 

 the external influences which prevail in the spring. The bud 

 period is therefore the sensitive period (mobilis cetas) of the 

 primordiimi under consideration.^ 



On the other hand, the plasticity of the primordium breadth 

 of the falissade cells depends almost entirely on the external 

 conditions which prevail in the spring. Its sensitive period 

 corresponds, therefore, to the vernal period. 



The properties length and breadth of the leaf-blade may be 

 looked upon as being primordia. They are, however, in reality 

 compound properties, which depend on the number of cells and 

 on their dimensions. As a consequence of this the sensitive 

 period of the mentioned properties extends over both parts of 

 the period of development of the leaf.^ 



The knowledge of the sensitive period of the primordia is 

 important for embryology in general, and especially for the 

 investigation of plasticity, adaptation and correlation between 

 plastic properties. 



A curious example of modified correlation in relation with the 

 sensitive period has been described by DE VRIES. In Papaver 

 somniferum, subsp. polycephalum, the central (normal) pistil is 

 surrounded by a number of smaller supplementary pistils. In 

 ordinary specimens an evident correlation exists between the 

 dimension of the central pistil and the degree of development of 

 the supplementary pistils, a large central pistil coinciding 

 with well-developed supplementary ones, and vice versa. The 

 sensitive period of the property supplementary pistils coincides 

 with the early period of the life of the individual, whereas the 

 sensitive period of the property dimension of the central pistil 

 coincides with a later part of the individual Ufe. If a specimen 

 is brought under unfavourable conditions of existence (poor 

 food) during the first weeks, the supplementary pistils are 

 hampered in their development. If the same specimen is 

 brought (at the age of about six weeks) under better condi- 

 tions, a large central pistil may still be obtained. In this way a 

 large central pistil coincides with incompletely developed 

 supplementary pistils, the ordinary correlation being entirely 



1 The sensitive period idea has been introduced in botanical science by DE 

 VRIES. 



" The sensitive period of several properties of the leaves of Fagus silvatica 

 with reference to the influence of light and shadow has been investigated by 

 Dr DE BOIS (Botanic Garden at Ghent). 



