Nos. 451-452.] ^vV^mJ/F OF THE CONIFERALES. 



the ruptures which they not infrequently exhibit (Fig. 35), and 

 in the various developmental stages which may be seen not 

 infrequently (Fig. 34). These show that resin gathers locally 

 upon the inner face of the trnchcid wall, and as its volume 

 increases it projects toward the ccntrr Imm all sides, until com- 

 plete coalescence occurs. 



The facts thus obtained proved most conclusively that the 

 transverse plates are obviously resinous and not o|" the nature 

 of cellulose, even partially, and the conclusion a])i)cai-cd to be 

 justified that they consist of gum dannnar, but of a highly modi- 

 fied and highly refractory character. The same evidc lu e also 

 conclusively shows that the cells in whieh the plates are de\cl- 

 oped are normal wood tracheids and not wood pareneh) nia, which 

 is altogether unknown in both Araucaria and Agathis, within the 

 limits of the investigated species. 



We are naturally led to ask what is the purpose of these resin 

 plates? The peculiar form in which the resin is deposited, 

 and the particular location of the plates, points with much force 

 to their connection with some functional activity, since if it were 

 simply a question of storage of the secreted products the latter 

 would hardly be disposed as found but rather after the manner 

 common to so many of the Cupressineaj ; and this suggestion 

 gains strength from the fact that both in the particular form of 

 the resin masses and their location in the tissue, Araucaria 

 and Agathis are peculiar among all allied genera. No exact 

 comparison can be estabhshed with other plants, and it is dif- 

 ficult to suggest an explanation which is adequate. One thing 

 does seem clear, however, and that is that since these plates are 

 of an impervious nature and developed, in some eases at least, 

 in connection with a special constriction of the traeheid cavity, 

 they offer, and possibly they are specially designed to afford, a 

 definite obstruction to circulation in a vertical direction. In 

 this sense they may be designed to serve the same general 

 purpose that is accomplished by the development of thyloses in 

 the vessels of the angiosperms, or in the resin canals of the 

 higher Conifers. It is therefore possible that they may be 

 connected in some way not at present clear, with a more com- 

 plete restriction of the circulation to a radial direction, and 



