1898-99.] MORPHOLOGY OF THE CENTRAL CYLINDER IN THE ANGIOSPERMS. 631 
In the reduced central cylinder of certain angiospermous species of 
aquatic or amphibious habit, the stelar system has become contracted 
on account of the correlated degeneracy of the vascular elements. In 
these cases, the parenchymatous core in the centre of the stele, ¢,g., that 
of Parnassta palustris, represented in photograph 25, plate 11, is to be 
regarded as included pericycle, morphologically similar, for example, to 
that found in the young fibro-vascular axis of Przmula farinosa. The 
contracted central cylinder in such cases, is none the less to be regarded 
as essentially siphonostelic, although it has no true medulla. 
IV.—We may now ask if the study of the development of the central 
cylinder in the Angiosperms supplies any morphological facts which are 
of phylogenetic value. Attention has already been directed to the 
primitively tubular character of the central cylinder throughout the 
Angiosperms. It has been furthermore noted that the stelar tube is 
characterized by foliar gaps corresponding to the traces of the leaves. 
It is of intere.t to discover whether the occurrence of foliar lacunz is 
constant, or whether it is in any way influenced by environment. The 
Cactaceze and the amentiferous genus Casuarina present perhaps the 
most extreme cases of foliar reduction among the Angiosperms, and it 
is of interest to discover whether the foliar gaps are obsolete in 
these examples. . The writer has satisfied himself by examination of 
species of Opuntia and Cereus, of the accuracy of Ganong’s” statement 
as to the occurrence of gaps in the fibro-vascular system of the Cactaceze, 
corresponding to the extremely reduced leaves. The stelar system of 
seedlings of Casuarina equisetifolia grown from seeds obtained from 
Kew, is also characterized by the presence of well-marked foliar gaps. 
‘It may consequently be assumed that the occurrence of foliar gaps is a 
constant characteristic of the Angiosperms. 
The possession of an essentially tubular stelar system interrupted 
invariably by foliar gaps appears to be a palingenetic feature of the 
Angiosperms. In this group there are so few anatomical characters which 
can rank as phylogenetic criteria that the demonstration of an additional 
one may be expected to contribute something to the solution of the 
extremely difficult problem of its origin. 
In an earlier memoir,” the writer has called attention to the value of 
_ a study of the stelar system in connection with the phylogeny of vas- 
cular Plants. It would indeed be strange if an apparatus so character- 
istically separating them as a whole from the lower Cryptogams did not 
52. Beitrag. Z. Kennt, d. Morph, u. Biol. d. Cacteen. Inaugural. Dissertation, p. 13, etc. 
53- Mem. Boston Soc. Nat. Hist., Vol. 5, No. 5. 
