D. PILOSUS, AND THEIR NATURAL RELATIONSHIP. 235
becomes apparent. D. sylvestris goes on with new and vigorous
developments ; out of the sheath of the petiole is developed the cup,
and each pair of leaves is strengthened and rendered rigid by a more
perfect system of venation than obtains in D. pilosus. The prickly
system is afterwards matured with the same vigour, and the whole
gives rise to a very robust plant having flowering heads proportionately
much larger than its kinsman. There is still usually an excess of
vigour expended in different ways, sometimes in producing very
long and foliaceous bracteoles, sometimes in bifurcating a leaf or
leaves, and sometimes in producing an additional leaf or pair of
leaves at one of the upper nodes.
In D. pilosus this vigorous growth is pretty well absent through-
out. In this species, in place of the cup, there is developed from
the sheath of the petiole, which is not greatly expanded, a fringe of
bristly hairs, and also some prickles on the earlier nodes of the stem.
The cauline leaves, which are the largest of the plant in both species,
in D. pilosus are peculiar. They have a naked petiole for the most
part, but at the base of the leaf-limb some two or three leaflets are
generally developed. The leaf, moreover, has a flabby appearance.
It is at this stage that the essential weakness of the plant, as com-
pared with D. sylvestris, becomes apparent. The flower stalks are
clothed with weak prickles, and, with the flower-heads, attain a size
comparable with D. sylvestris only when its stem has been cut
nearly through so as to allow only two or three bundles of fibres to
nourish the plant.
We have adverted to the not infrequent case of bifurcation (or
even addition) of a leaf of D. sylvestris, which, of course, means that
the mid-rib of the leaf separates at a certain point into two equal or
unequal portions, and these develop proportional independent
leaflets. This must be regarded as the most pronounced stage of a
phenomenon which is traceable in both species, but far more
frequently in D. sylvestris. It seems reasonable to suppose that the
plant exercising this function (of variation) most readily should be
the newer form.
Closely associated with the fibro-vascular tissue giving rise to these
variations are the prickles, and some attention should therefore be
directed to their structure and distribution. In the advanced stage
of D. sylvestris they almost acquire the consistency of spines,
whereas in D, pilosus they are often represented by hairs. They are
all epidermal, and there is no material difference between the hair of
Q 2