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Elementary Biology. 



sary to go round the stem before coming to the leaf directly 

 above that from which the start is made (following the bases 

 of successive leaves on the branch in regular order), and if 

 the denominator represent the number of leaves passed, then 

 the numerators of two successive fractions added together 

 give the numerator, and the denominators added together 

 the denominator of the succeeding fraction. Thus the 



FIG. 99. DICOTYLEDONOUS LEAF. (Thom.) 



simplest spiral phyllotaxis must be that where the leaves are 

 alternate, one on one side and the other on the other side 

 of the stem. Manifestly, under such circumstances, we 

 must pass two leaves and go once round the stem to reach 

 the leaf directly above that from which we started. Hence 

 we have here a \ spiral. Again, still circling the stem once, 

 every fourth leaf might be that directly above the starting 

 point. The fraction then would be ^. According to the 



