248 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
that the zone does not close up after having early parted with 
a cord for an appendicular organ. I think we may attribute this 
failure to a want of activity in the formation of wood, which 
may be correlated to the insufficiency of foliage during the hot 
months*. Perhaps the curious cavities described + by Prof. 
D. Oliver in a species of Acantholimon as occurring in the wood 
may be attributed to the same cause. 
As other examples of anomalous stems, Dr. Volkens describes 
and figures that of @ypsophila Rokejeka: the pericycle (which is 
often very active in the Caryophyllex) puts on a very dense zone 
of thick-walled sclerenchyma, in lieu of xylem, which in this 
plant is comparatively feeble. A similar result occurs in Tele- 
phium spherospermum, of the allied order Paronychiacee. I have 
found an analogous result of pericycular activity in Iphiona 
mucronata. Tuside a cortex there are zones of square, oblong, or 
irregular shaped patches of sclerogen imbedded in a paren- 
chymatous tissue t, and only isolated patches of xylem around 
the medulla, all being imbedded in a “stereome-zone.” <Astra- 
galus Forskalii has a very anomalous stem. It has large cords 
of liber outside a zone of cork, then another series outside the 
phloem, a third, complete zone in the middle of that tissue. An 
anomalous xylem follows surrounding a medulla in the centre of 
which is a column of collenchyma! To this I would add the 
stem of Anabasis articulata, which has only rudiments of leaves, 
opposite and distichous. All the elements, as seen in a transverse 
section, make up four series of crescent-shaped structures around 
an hourglass-shaped pith. Taking a “ wedge,” there is scleren- 
chyma forming the angle, then a zone of large vessels and wood- 
fibres, then cortical parenchyma, and sclerenchyma outside. 
It may be observed that the four wedges, roughly resembling a 
Maltese cross, here appear to be correlated to the positions of 
the rudimentary leaves; the xylem tissues failing just were no 
leaves occur. 
As long as the shoots are young and green, one may add to 
* TI find that Grisebach confirms this idea, for in speaking of the partially 
developed wood in Haloxylon Ammodendron he says :—“ La suppression du 
feuillage détermine la croissance incompléte du corps ligneux” (La Vég. du 
Globe, i. p. 630). 
t L.c, tab. 51. fig. 24, 
} Dr. Volkens figures a section of a young stem; but does not appear to have 
noticed the development of pericycle. 
