[PENHALLOWJ A BIRCH ROPE 247 



pertraphy originating immediately beneath it, and it seems to offer the 

 most acceptable explanation of some of the most important features of 

 the growth. 



It is evident, however, that after emergence, the tumour must have 

 continued its development in the form of parenchyma tissue until the 

 completion of its length, and that this development must have been con- 

 tinous within a short period as already shown. That there was a certain 

 definite, though limited necessity for resistance to transverse strains such 

 as would be developed by wind pressure, is evident from the presence 

 of mechanical tissue in the form of the cylindrical, branching and thick- 

 walled parenchyma cells ; but that the necessity for such mechanical ele- 

 ments was not great, is also implied by its limited extent and by the com- 

 plete absence of fibrous tissue. 



The development of so attenuated an organ of such great length, 

 necessitated some special provision for the transport of nutrient ma- 

 terials, as well as for the aeration of parts. This latter we have seen to 

 have been provided for through the large and numerous intercellular 

 spaces of the parenchyma tissue, and it is possible even, that the central 

 cavity may have developed before complete maturity with special refer- 

 ence to this particular purpose. The longitudinal circulation of the nu- 

 trient fluids was provided for in the first instance, by the parenchyma 

 cells themselves, but this provision no doubt proved inadequate after a 

 time and it was therefore supplemented by the mechanical cells which 

 also served as conductive cells, as indicated by their strongly developed 

 and numerous pits. Lateral circulation was provided for in the same 

 way and through the same elements, but more largely by the develop- 

 ment of elementary medullary rays which served to connect the central 

 region with the outer regions where the formation of cork was particu- 

 larly active; and it is quite probable that these structures also served to 

 some extent for lateral aeration. 



We have yet to gain an explanation of the way in which the gro^vth 

 in length was accomplished, it may have been brought about in one of 

 three ways, or by a combination of two or more of them. 



1. The development may have been wholly basal and confined to 

 the tissues lying within the area of the living bark of the parent tree. 

 In that case the growth would have been pushed forward, and the in- 

 crease of length would have depended entirely upon the formation of 

 new tissue at the base. Had such a method of development obtained, 

 the tumour wf^uld probably have shown more or less marked irregularity 

 of surface indicative of want of absolutely continuous development, but 

 nothing of the kind appears. Moreover, such basal development would 

 have resulted in the cork being disposed in transverse layers, inasmuch 



Sec. IV., 1906. 19 



