520 IOWA ACADEMY OF SCIENCE 



It is interesting to note, in comparing the series of sections, 

 the presence of dichotomy, as manifest in the double vascular 

 bundles, and its disappearance in the higher conifers, — also the 

 development of palisade parenchyma in all flat leaves. Some- 

 thing of phyletic relationship is shown in some of the sections, 

 as for instance, the deeply infolded parenchyma walls of the 

 Larches, linking them with the preceding Pines; with the de- 

 crease of the same character to mere corrugations in the paren- 

 chyma walls of some of the Spruces. The presence and de- 

 crease of a conspicuous bundle sheath as advance is made through 

 the group, is also notable. 



Note: Throughout the descriptions it has been the aim to 

 describe all the characters seen in section. Most reliance, how- 

 ever, can be placed in the appearance of such characters as the 

 vascular bundles, bundle sheath, resin ducts and hypodermai 

 cells, as they show most constancy. 



The characters of the parenchyma and the pithlike cells of 

 the vascular bundles can not be considered as reliable, though 

 the infolding of the walls of the former appear to be uniform. 



Throughout the Plates it has been the aim to use uniform 

 means of portrayal. The shaded space without the epidermal cells 

 represents cutin except where this is very thin, where only a line 

 is used to represent its boundaries. In representing the phloem 

 in the bundles single lines are used to distinguish it from the 

 thick walled xylem though in many cases the walls are as thick 

 as those of the xylem cells, though not lignified as is the xylem. 



The numbering of the figures of the plates is the same approx- 

 imately in each case — Figure 1 shows the leaf section as a whole. 

 Figure 2, section of epidermis. Figure 3, section of resin duci 

 if present. Figure 4, section of vascular cylinder. 



