Anatomy and Histology. 105 



seasons of growth. Comparing the two cases, we find that the structure 

 attained in the cortex by a seedling of one season may be the same as that 

 attained in three years by one of slower growth, while the number of 

 growth-periods is reflected , albeit frequently only indistinctly , by the wood . 

 It is, however, generally true that the ring-structure may be made out. 



EPICOTYL. 



Seedlings partially etiolated by being grown under a muslin screen, 

 in which the internodes have lengthened, render the analysis of the tis- 

 sues easier. The lowermost internodes of such seedlings receive primarily 

 six bundles (plate 24, fig. 5) from the hypocotyl, but the number is at 

 once increased, so that immediately above the base eight or even more 

 bundles may be counted (plate 24, fig. 6). The increase is more marked 

 in plants with short internodes, and the primary condition is quickly 

 masked. The development of the stereome which arises in the primary 

 leptome is in the primary numerical relation, there being at first six 

 bundles, opposite the median and lateral leaf-traces of the first two foliage 

 leaves. These relations are shown very beautifully in a section taken 

 from a seedling which had developed one-sidedly, and this is figured in 

 plate 30, fig. 2. The relations of the primary cortical canals received from 

 the hypocotyl are well shown also in this section. One pair of these 

 accompanies the median leaf-trace of the first leaf, the other pair that of 

 the second leaf. The third petiole may receive two or one, and this is true 

 of all the earlier leaves as far as the tenth node, approximately. The 

 primary condition, that in which two lateral canals occur, may recur even 

 in later stages of development, but only infrequently. As they pass into 

 the leaf one becomes smaller and ends blindly (plate 38, figs. 3 to 9), 

 while the other extends into the leaf-blade. In this there is a striking 

 similarity between the earlier foliage leaves and the cotyledons, constitut- 

 ing a morphological argument against the theory that the cotyledons are 

 not primitively leaves. The absence of medullary 1 stereome, mentioned 

 above in the paragraphs dealing with the hypocotyl, will be noted, and 

 this condition, as in the case of retonos, persists until the level of the 

 tenth internode or thereabout. Sections of field seedlings with short inter- 

 nodes at a distance of several millimeters from the insertion of the coty- 

 ledons show no medullary stereome, and this is true also of medullary 

 canals. 



The same section (plate 30, figs. 3, 4) serves, in addition, to show 

 very clearly the origin of the periderm, which in the definitive stem, as in 

 the earlier internodes, occurs in the first or outermost cortical layer of cells 

 (as shown by Ross, 1908). Fron and Francois (1901) state differently, 

 and their drawing depicts the earliest suberogenous divisions in the 

 second layer of cells; in this, however, they are in error. Their drawing is 

 taken from a section through the base of a petiole, as the position of the 

 leaf-traces, so labeled, shows. In such a section, it is true, the earlier 

 divisions will be seen in the second, third, or even fourth layer. I have 

 introduced two figures taken from portions of the tissue in question on 



1 1 use this in a purely descriptive sense. " Perimedullary stereome" has been 

 used. The origin of this stereome is dealt with beyond (p. no). 



