156 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



fully developed 1 . The primary leaves of Vicia Faba differ greatly in 

 configuration from the succeeding ones ; they are reduced to small three- 

 toothed leaflets, the middle tooth representing the leaf-blade, the lateral 

 ones the stipules. The primordium of the leaf has remained stationary 

 here at a very early stage, and in subsequent leaves experiences only an 

 increase in size and no further morphological differentiation takes place. 

 We can prove this experimentally. The axillary shoots which spring from 

 the base of a plant all possess the same form of leaf. If the chief shoot 

 be removed above the primordium of a lateral shoot, this will be forced 

 to shoot out at an early period, and instead of the primary leaves there 

 will be found upon it according to the degree of development to which it 



VI 



FIG. 94. Vicia Faba. Primary leaves. AY normal primary leaf from the base of a chief axis. 7- VIII different 

 stages of transformation of primary leaves at the base of a lateral shoot, obtained through the removal of the chief 

 shoot ; a leaf-apex, st stipule or its vestige, b lateral pinnule. The venation is not completely represented. 



had already attained the most varied intermediate forms between the 

 primary leaves and the foliage-leaves or typical foliage-leaves (Fig. 94). 



This kind of case shows us also that the developmental arrest of the 

 primary leaves stands in correlation with the formation of other organs of 

 the same plant. It is clear moreover that the leaves which are first 

 developed upon a plant and which stand near the ground can more 

 easily bear the want of a higher differentiation than the following ones 

 which are exposed to the wind, rain, and other external factors. 



These differences in external relationships appear still more strikingly 

 when we look at groups of plants marked by common biological features. 



1 Further examples will be found in Flora, 1889, p. 29. 



