COMPARATIVE ANATOMY OF ANGIOSPERMS 315 



gaps is the ancestral condition in the Monocotyledons. In some 

 cases, e. g., Symplocarpus foetidus, the pith and cortex are 

 continuous in the seedling through the foliar gaps, although 

 they no longer appear to be so in the adult. An internal endo- 

 dermis or stelar boundary is also sometimes present in the young 

 plant, but has usually quite disappeared in the adult. 



The typical bundle of the Monocotyledons is amphivasal 

 concentric. Such a bundle is shown in Fig. 113, HH. In this 

 type of bundle the tracheary tissue surrounds the phloem, and 

 not the phloem the tracheary tissue, as is generally the case in 

 the Pteridophyta. The amphivasal concentric bundle is char- 

 acteristic of the Monocotyledons from the grasses (Zizania, 

 etc.) to the orchids (Ilabenaria, Cypripedium, etc.), and is 

 quite as constant a feature as the scattering disposition of the 

 tibrovascular strands. This type of bundle resembles the am- 

 phicribral concentric bundles of the Pteridophytes in showing- 

 no evidence of secondary growth. Amphivasal strands are ab- 

 sent in the leaves and reproductive axes of the Monocotyledons, 

 and generally in the seedlings as well. Unlike the concentric 

 strands of the Gymnosperms, they are accordingly a cenogenetic 

 and not an ancestral feature, but on account of their widespread 

 occurrence in the group have an important phylogenetic signifi- 

 cance. 



Secondary growth has been supposed to be entirely lacking 

 in the collateral strands of the Monocotyledons, but Queva 12 

 has recently shown that characteristic secondary growth is 

 present in the bundles of the tuberous base of the stem of 

 the liliaceous genus Gloriosa. The activity of the cambium 

 becomes apparent during the season after the formation of the 

 tuber, when it is passing its reserve products into the aerial 

 stem. From the occurrence of a cambium in Gloriosa, etc., 

 Queva has drawn the conclusion that the Monocotyledons are 

 derived from the lower Dicotyledons. 



The most salient anatomical features of the Monocotyledons 

 are the scattering disposition of their closed tibrovascular 

 strands, and the presence of amphivasal concentric bundles. 

 These features, although practically universal, are not primi- 

 tive; for a study of the leaves, reproductive axes, and seedlings 

 shows often a dicotyledonous disposition of the generally col- 

 lateral strands. Hence we may infer that the Monocotyledons 



