Ch. IV, 5] ARRANGEMENTS OF LEAVES 



143 



seem to possess in case of the opposite arrangement and the 

 lower fractions of the spirals vanishes in case of the higher 

 systems, which are inappreciably different in so far as leaf 

 exposure is concerned; while, moreover, the ultimate ex- 

 posure of leaf blades is determined chiefly by their own photo- 

 tropic movements on their petioles, with little or no regard 

 to the plan of their origin. Later studies, however, seem to 

 show that phyllotaxy originates in the construction of buds, 

 as an incidental re- 

 sult of the order in 

 which the nascent 

 leaves develop in 

 relation to one an- 

 other upon the cone 

 of embryonic stem 

 tissue. This order 

 of development, in 

 turn, seems to be 

 connected with con- 

 ditions of mutual 

 pressure of the form- 

 ing leaves upon one 

 another in buds of 

 different shapes, this . FlG - ^ ~ A * ead ° f , Sunflower in . seed ; 



r ' showing its symmetry, which is an expression of 



pressure manifesting phyllotaxy. (Drawn from a photograph in the 

 itself verv differ- ^ eport °f ^ e New Jersey Experiment Station for 



ently in slender 



buds, which mostly produce the opposite system and low 

 fractions, as compared with the broad or flat buds, which 

 chiefly produce the higher fractions. Apparently the leaves 

 originate regularly and successively in the lines of least 

 resistance in the differently shaped buds, — and the result 

 is phyllotaxy. Herein we seem to have a particularly clear 

 of one of those purely structural factors which were 

 earlier mentioned as having a part with adaptation and 

 heredity in determining details of plant form. 



