GERMINATION AND HISTOLOGY OF WELWITSCHIA MIRABILIS. 2] 
of the stem ; and their cell cavity has been almost com- 
pletely obliterated, so that they only remain as occasional 
irregular masses of lignified wall between the parenchy- 
matous cells. In longitudinal section the thickening is 
seen to be drawn out into a loose spiral, or sometimes 
almost into straight lines. The bearing of this arrange- 
ment of the protoxylem will be seen when the course of the 
fibro-vascular bundles at the apex of the stem has been 
traced. 
The constituents of the xylem of the hypo-cotyledonary 
stem are the same as described for the leaf bundle.! It will 
be noticed that, as we pass from the xylem, through the 
cambium, to the phloem, the tissues are arranged in regular 
rows, pointing to a development from a cambium layer 
which, as was seen (fig. 13), makes its appearance very 
early. <A group of sclerenchymatous fibres skirts the fibro- 
vascular bundle on the central side (f, fig. 14). Lying 
between these and the cambium is the soft bast. Here, as 
is found in the bundles of older leaves, the study of the 
tissues is rendered difficult by the tendency of the cell walls 
to swell. Nevertheless, in longitudinal sections treated 
with Schultz’s solution, it was possible to trace a structure 
corresponding to that in the phloem of other gymnosperms.” 
Particularly worthy of notice is a lateral extension of the 
cambium layer, though variable in its time of appearance, 
and extent, it occupies a definite position in the stem (fig. 
11). In most cases observed, the layer is intermittent as in 
fig. 14. In no case were there found any vascular elements 
developed from this meristem. Under the present circum- 
stances it is therefore impossible to say what relation this 
cambium may bear to the process of secondary thickening of 
the hypo-cotyledonary stem. 
We may now pass to the plumule and cotyledons, and 
trace the course of the fibro-vascular bundles which proceed 
from them, till they merge into the four fibro-vascular 
bundles of the hypocotyledonary stem. 
It has been previously stated that there are four main 
bundles in each cotyledon, to which all the other bundles 
attach themselves. From each of the cotyledons accord- 
ingly four bundles enter the stem. This may be seen in 
fig. 15 a. Towards the base of the cotyledon these 
bundles arrange themselves in pairs, and finally, after 
1 Cf. De Bary, ‘ Vergl. Anat.,’ p. 348. 
‘Loe. cit., p. 188. Bertrand (“ Anatomie des Gnetacées, &c.,” ‘Ann. 
d. Sci. Nat.,’ Série y—xx) has also described “ cellules grillagées” in the 
phloem. 
