22 2 BOTANICAL GAZETTE [skptember 



most ancient of the conifer group), the mesarch structure of the 

 bundle is as pronounced and well developed as it is in the case 

 of the cycads or Medulloseae. As regards these two latter 

 groups, the writer regards their mesarch bundles as derived from 

 the splitting up of one or more concentric bundles like that of 

 Lyginodendron. In any case the mesarch collateral bundles of 

 the foliar organs of all these plants are to be regarded, like 

 those of the stem, as reductions from the concentric type. But 

 this concentric type actually occurs here and there in the sporo- 

 phylls of the cycads. 



On arriving at the level of the angiospermous plants, all trace 

 of any vestige of the old concentric type of bundle in the form of 

 a mesarch structure has completely vanished both in the stem and 

 in the leaf, the purely endarch structure prevailing everywhere. 

 The fact of this type of structure occurring well-nigh universally 

 in these plants which, in other parts of their organization, are seen 

 to be the most advanced in evolution of all plants, is an indication 

 that this type of vascular structure is also to be regarded as the 

 most advanced, because the most advantageous and the best 

 adapted to the requirements of the plants exhibiting it. The 

 writer, however, considers that the conclusions drawn by Jeffrey^ 

 from the study of the vascular cryptogams, as to the origin of 

 the vascular tissue of the stem are rather too hastily applied to 

 the case of the angiosperms. We are not yet in a position to 

 say whence the vascular system of these latter plants was- 

 derived. All we dare surmise is that, looking far enough back,, 

 it is probable that the vascular tissue of the angiosperms had an 

 origin similar to that of the forms below them in the scale. 



The cause of the disappearance of the primitive mesarch 

 structure can be seen in tracing the evolution of the vascular 

 tissue from such forms as the Medulloseae upward. Obviously, 

 the most economical method of increasino: the amount of vascu- 

 lar tissue for purposes both of conduction of w^ater and food 

 substances and of resistance to bendinof strains would be by 

 the addition, through the means of a cambium, of new tissue 



ife 



cylinder; now, in pro 



