702 PEOCEEDINGS OF THE ACADEMY OF [Oct., 



discover the cause which has produced this absohitely unique kind of 

 palm-stem. 



The explanation is found in the manner in which palm stems grow 

 in general and the influence of the meteorologic and soil (edaphic) con- 

 ditions on the cells of the growing crown. It is a well-known fact 

 that the primary root of all palms soon perishes and is replaced by ad- 

 ventitious roots springing from the base of the stem. It is some years 

 before the stem appears above the surface of the ground. In the mean- 

 time, the circumference of the growing point is continually increas- 

 ing, producing successively larger leaves, so that the much-compressed 

 axis forms an inverted cone, which is kept in position by the numerous 

 adventitious roots. Finally, a rosette of normal-sized leaves is pro- 

 duced and the stem grows erect, forming a cylindric structure, the 

 diameter of which varies widely in different species, but which, once 

 formed, shows no secondary increase in thickness by the formation of 

 new elements, so that the diameter of the stem remains almost uni- 

 formly the same from the base to the top of the tree. There is, how- 

 ever, an increase in diameter in the older stems of some palms which 

 causes the gradual tapering upwards which is sometimes observed. 

 This increase is due to the expansion of the parenchymatous fun- 

 damental tissue which separates the vascular bundles, accompanied 

 by an increase in the cell-cavity and in the thickness of the walls of 

 the sclerenchymatous fibers which support the bundles. 



Evidently, we can explain the hour-glass caudices in the Bermuda 

 palmetto by the general application of principles of palm-stem growth 

 described above. If we remember that the diameter of the cylindric 

 palm-stem is determined by the dividing cells of the rosette, or crown 

 of green leaves, then any environmental condition which influences 

 the growth and division of this terminal mass of meristematic cells 

 influences in a direct waj^ the diameter of the stem. We have already 

 alluded to the fact that the leaves of the Bermuda palmetto in ex- 

 tremely rocky and dry situations are j^ellowish-green and smaller 

 than those of a tree in correspondingly rich soil. Fig. 9 perhaps 

 best represents a tree found in an extremely dry situation with 

 a trunk that gradually tapers upward, showing that in youth the 

 tree made a rapid and vigorous growth, but that in subsequent years 

 the conditions which have influenced the crown growth of this par- 

 ticular tree have been unusually unfavorable. Now, if we apply this 

 discovery to a study of other trees, many forms of which are figured 

 on Plate XLII, we have the solution of the question concerning the 

 apparent anomalous condition of the stem structure. 



