314 MORPHOLOGY OF ANGIOSPERMS 



as companion cells. Companion cells are quite absent in the 

 Gymnosperms, but Strasburger has pointed out that here the 

 marginal cells of the medullary rays perform the physiological 

 function of companion cells. 



The Dicotyledons as a group are distinguished anatomically 

 from the Gymnosperms by the entire absence of palingenetic 

 pteridophytic features of any sort in the fibrovascular tissues 

 of their stems and leaves. The bundles are throughout endarch 

 collateral, except in the root, where they are exarch, as in all 

 other vascular plants. The concentric bundles which occasion- 

 ally occur in the Dicotyledons are obviously cenogenetic, and 

 have no phylogenetic significance. Both the xylem and phloem 

 of the Dicotyledons show a marked advance in differentiation 

 over all the Gymnosperms. The central cylinder of the stem 

 in the Dicotyledons is characterized by the presence of foliar 

 gaps, and accordingly, if the Dicotyledons are to be regarded 

 as derived ultimately from pteridophytic ancestors, as appears 

 to be the case, their descent is apparently from the Filicales, 

 either directly or through some living or extinct phylum of the 

 Gymnosperms. The argument for descent from a gymnosper- 

 mous ancestry seems to gain great force from the entire absence 

 of pteridophytic features in the shoot or leaves of the dico- 

 tyledonous Angiosperms. 



MONOCOTYLEDONS 



The arrangement of the bundles in the adult stem of the 

 Monocotyledons is very characteristic. Instead of being dis- 

 posed in a circle, as in the Dicotyledons, they are scattered 

 throughout the central cylinder. Fig. 113, FF, illustrates this 

 peculiarity. Not infrequently, however, e. g., in the Lilia- 

 ceae, the bundles are obviously segments of a fibrovascular tube, 

 just as is typically the case in the Dicotyledons. Fig. 113, GG^ 

 shows this feature in the rhizome of Clintonia borealis. Sub- 

 tending gaps between the bundles are to be seen smaller fibro- 

 vascular strands, which are leaf-traces. In this example we 

 have obviously to do with a fibrovascular tube with foliar gaps. 

 Interestingly enough, the tubular arrangement of the fibrovas- 

 cular elements is frequently present in monocotyledonous seed- 

 lings, although characteristically absent in the adult. Hence 

 it may be inferred that the tubular central cylinder with foliar 



