EXPANSION OF THE BARK 69 



is, in this connection, that nature is not always 

 formal. There is no plant-form so rigid that it 

 may not be broken or modified iipon occasion. 

 Neither is it true that the buds or leaves of all 

 plants have a definite method of arrangement, 

 although such is the rale. The pupil will be 

 puzzled to work out cycles in the stems of 

 many lilies, for example. 



Suggestions. — In the study of phyllotaxy, it is not only im- 

 portant to work out the mathematical cycles, but also to discover 

 deviations or variations from the rule. In other words, the pupil 

 should examine any branches upon a tree rather than those only 

 which best show the mathematical arrangement. He should en- 

 deavor to discover whether deviations are in any way associated with 

 the position of the branch with respect to light. He will be in- 

 terested in an endeavor to find a numerical arrangement of the 

 eyes of an Irish potato. 



XIII. EXPANSION OF THE BAEK 



72. The fasciated branch in Fig. 67 not only 

 raises the question as to the cause of fasciation, 

 but the more pertinent one as to why stems re- 

 main normally terete (or cylindrical). Consider 

 for a moment the general make-up of the plant 

 cylinder. The young shoot is tightly enveloped 

 with bark. We observe that in many plants the 

 increase in diameter of the stem comes about by 

 the formation of rings of new tissue (or new 



