408 WALTER GARDINER. 
whole, is basipetal, each pair of lateral lobes arise and grow 
from the basal meristematic zone in an apical manner, so 
that their development is quite similar to that of the primary 
lobe (fig. 3). Whilst the apical growth of the terminal 
lobe is still proceeding a strand of cells is differentiated in 
the median line of the leaf; they are small and elongated, 
their nuclei are very conspicuous and elongated in form, 
their protoplasm is granular and stains deeply with hema- 
toxylin, and their cell walls are very delicate. The differ- 
entiation proceeds from the base towards the apex, although 
it may be said to be almost simultaneous. This strand of 
cells is evidently a procambium. 
At the same time that this differentiation has been going 
on in the leaf, strands of cells have become differentiated in 
the stem, just behind the punctum vegetationis, in such a 
way that the apex of one of these strands of the stem is on 
a level with the base of the leaf, and by a differentiation in 
the few intervening cells the procambium of the stem comes 
to be continuous with that of the leaf. 
Like the primary lobe, the lateral lobes consist at first of 
meristem covered by a dermatogen. Here, too, the procam- 
bium becomes differentiated as strands of similar cells, 
exactly resembling those first described. The procambium 
of a lobe may be directly continuous with that of the main 
leaf-bundle, or may join with one or more strands from con- 
tiguous lobes before so doing. The differentiation of primary 
meristem into procambium, in fact, does not, as a rule, 
follow any definite course, but starts at particular centres 
and in various ways, thus giving rise to the reticulate dis- 
tribution of the fibro-vascular bundles in the fully-developed 
leaf. 
The next step is the conversion of the procambium into 
permanent tissue. A single row of spirally thickened protoxy- 
lem cells makes its appearance in the centre of the median 
procambium strand. ‘The further conversion of the meriste- 
tematic into permanent tissue takes place centrifugally. 
There is a definite but slight increase in the number of the 
elements of the main fibro-vascular bundle from the apex 
towards the base. The zone of meristem cells at the base 
of the leaf is traversed by the primary fibro-vascular bundle, 
and as the lateral bundles are differentiated they become 
continuous at their proximal extremities with the peripheral 
portion of the primary fibro-vascular bundle, which is still 
of the nature of procambium. The fact that the conversion 
of procambium into permanent tissue takes place in a cen- 
trifugal manner allows of cell fusion so long as any procam- 
