376 



RELA TION TO ENVIRONMENT. 



Fig. 428. Fig. 429. 



Fig. 428. Shoot of butternut 

 showing leaf -scars, axillary buds, 

 and adventitious buds (buds com- 

 ing from above the axils). 



Fig. 429. Shoot and bud of 

 white oak- 



numerator denotes the number of turns 

 around the stem in each cycle, and the 

 denominator the number of leaf-scars in 

 the same distance. In a general way we 

 find in plants only such arrangements as 

 are represented by the fractions given 

 above. These fractions show the curious 

 condition that the numerator and de- 

 nominator of each is equal to the sum 

 of the numerator or denominator of the 

 two preceding fractions. Much specula- 

 tion has been indulged in regarding the 

 significance of these definite laws of leaf- 

 arrangement. In part they may be due 

 to the desire that each leaf receive the 

 maximum amount of light. Only certain 

 definite geometrical conditions will insure 

 this. More likely it is due to the economy 

 of space alotted to the leaf-fundaments 

 in the bud. Here, again, geometiical 

 laws govern this economy. The phyllo- 

 taxy is nearly constant for a given species. 



734. Buds. The growing point 

 of the stem or branch together with 

 its leaf or flower fundaments and 

 protective structures is termed a 

 bud. Winter buds on woody plants 

 are terminal when inclosing the 

 growing point of the main axis of the 

 twig; lateral when the growing point 

 is that of a branch of the main 

 axis. Lateral buds are always axil- 

 lary, i.e., situated on the upper angle 

 between a leaf and the main axis. 



735. Buds occupying special po- 

 sitions. Several species of trees 

 and shrubs produce more than one 

 bud in each leaf-axil. The addi- 

 tional ones are termed accessory or 

 supernumerary buds. These may 



