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In fact, as the majority of parasites take their nourishment at the expense of the plants upon which they fasten themselves, they have no need, as a general thing, of elaborating through their foliar organs the materials that their hosts derive from the air; in a word, they do not breathe actively like the latter, since they find the elements of their nutrition already prepared in the sap of their nurses.
The winding of the foliar screw must curve the longitudinal ribs. The straighter and steeper the screw becomes, the more the ribs will become twisted. That this happens in the opposite direction is obvious, without further discussion. The twisting is the inevitable consequence of the reversal of the screw. Two points remain to be dealt with.
Nevertheless, from this statement by Mohl, I have ranked this species amongst the tendril-bearers; if classed exclusively by its foliar tendrils, it would be doubtful whether it ought not to have been placed amongst the leaf-climbers, with its allies, Fumaria and Adlumia.
Such partial malformations obey a rule, that is common to them and to other foliar enations, viz.: that the side of the leaf from which they emerge, is always their outer side. The inner surface of these enations corresponds to the opposite side of the leaf, both in color and in anatomical structure. The last of the four rules above mentioned is that of the homologous variability.
Mutisia is the only genus in the family, as far as I can learn, which bears tendrils: it is therefore interesting to find that these, though rather less metamorphosed from their primordial foliar condition than are most other tendrils, yet display all the ordinary characteristic movements, both those that are spontaneous and those which are excited by contact.
In both the sensitiveness is either confined to one side or extends all round the tendril. Both are either attracted or repelled by the light. The latter property is seen in the foliar tendrils of Bignonia capreolata and in the axial tendrils of Ampelopsis.
In the specimens examined by me all the tendrils were certainly foliar, and it is hardly credible that the same plant should produce tendrils of a widely different homological nature.
Union of the neighboring fruits and flowers on flower-heads, of the rays of the umbellifers or of the successive flowers of the racemes of cabbages and allied genera, seem to be rare. The same holds good for the adhesion of foliar to axial organs, of branches to stems and other cases of union.
The true leaves, if there are any, are reduced to mere spines or prickles on the surface, while the branches, in the prickly-pear and many of the ornamental hot-house cactuses, are flattened out like a leaf to perform foliar functions.
At its top, an abundance of the richest and most beautiful leaves spread out in graceful symmetry, and bend down on all sides, forming a figure like an umbrella; while the young leaf, still firm and compact in its foliar envelope, is seen standing erect in the centre of this foliage, like a lightning-conductor.
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