144 SCIENCE PROGRESS. 



and is hence more worthy to be taken as the morphological 

 unit of vascular tissue. It is indeed impossible to give a 

 morphological definition of a vascular bundle at all. " From 

 the very first those bundles which consist essentially of 

 definitely arranged groups of tracheae and sieve tubes . . . 

 have been called vascular bundles" (14, p. 232, Eng. ed.). 

 But thus defined, a "vascular bundle" has no constant 

 histological characters beyond the fact of containing both 

 xylem and phloem. According to the arrangement of these, 

 bundles have been classified as radial, concentric, collateral, 

 etc. Such an arrangement brings together vascular strands 

 of very different orders of complexity. In the first place 

 it associates the axial cylinder ("radial bundle") of a root, 

 possessing a number of quite distinct xylem and phloem 

 strands, with the "collateral bundle" of a Phanerogamic 

 stem, formed of a single strand of xylem and phloem in close 

 association, the latter being continuous moreover with a 

 portion only of the former. Again it associates even more 

 closely under the term " concentric bundle " the vascular 

 strands found in the stem of Auricula, Gunnera and Ferns 

 with those of quite different structure found in the pith and 

 cortex of Melastomacecz, etc. 



Such a classification is clearly, from a morphological point 

 of view, quite artificial. But if we extend the use of the term 

 bundle, as is often done, so as to include strands of tracheae 

 alone, and of sieve tubes alone, we can retain it as a con- 

 venient word without morphological connotation, and 

 applicable to any strand of tissue belonging to the vascular 

 system. And we may then qualify the word by any adjec- 

 tive we choose without morphological implication. Thus we 

 may speak of the composite radial bundle of the root as 

 composed of separate xylem bundles and phloem bundles 

 alternating at its periphery ; of the concentric bundle of the 

 stem of an aquatic plant as sometimes composed of separate 

 collateral bundles, in other cases consisting simply of a con- 

 tinuous cylinder of phloem surrounding a central strand of 

 xylem ; of the concentric bundle of a fern petiole gradually 

 passing to the collateral type as we trace it into the lamina, 

 and so on. Meanwhile the study of the homologies of the 



