206 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[35. 



The development of secondary vascular tissue takes place al- 

 most exclusively in such stems as are monostelic and in which the 

 primary bundles are common. It is clear that the additions to the 

 primary bundles in the older internodes of the stem, as well as any 

 secondary bundles which may have been formed from the cambium, 

 are not common, but cauline ; they are, however, in communication 

 with the primary common bundles of the young unthickened inter- 

 nodes which are bearing leaves; in fact, the newly-formed secondary 



vascular tissue of the lower 

 internodes of the stem is 

 in communication, on the 

 one hand with the root, 

 and, on the other with the 

 leaves ; and the channels 

 of communication between 

 root and leaf are main- 

 tained year by year by 

 the annual formation of 

 young conducting-tissue, 

 both wood and bast, in 

 the older parts of the 

 stem and of the root. 



It will be remarked 

 that the development of 

 secondary vascular tissue 

 takes place in those plants 

 the stems of which branch 

 more or less (e.g. an Oak), 

 while it usually does not 

 take place in those plants 

 the stems of which do not 

 branch (e.g. the Palm), or 

 do so only slightly. It 

 is obvious that, when the 

 stem is of branching habit, 

 the number of leaves must 

 increase year by year, whereas when the stem does not branch 

 the number of leaves does not vary materially. Hence the whole 

 matter may be summed up thus, that the development of 

 secondary vascular tissue in a stem is directly correlated with 

 an increase in the area of leaf-surface : as in each year the leaf- 



FIG. 154. Portion of a transverse section of the 

 stem of a Dracaena : e epidermis ; fc periderm ; r 

 primary cortex, with a leaf -trace-bundle l> ; x 

 merismatic zone in which new bundles g-g are in 

 course of development; m primary, and st second- 

 ary, fundamental tissue. (Magnified: after Sachs.) 



