=Formation of tubes.=--The production of ascidia or pitchers from the cohesion of the margins of one or more leaves has been already alluded to (see pp. 21, 30), but there is another class of cases in which the tubular formation is due, not so much to the union of the margins of a leaf as to the disproportionate growth of some portions as contrasted with others, whence arises either a depressed cavity, as in the case of a leaf, or an expanded and excavated structure, when the stem or some portion of it is affected. The fruit of the rose, the apple, the fig, and many others, is now generally admitted to be composed externally of the dilated end of the flower-stalk in which the true carpels become imbedded. Between such cases and that of a peltate leaf with a depressed centre, such as often occurs, to some extent, in _Nelumbium_, there is but little difference. In cabbages and lettuces there not unfrequently occurs a production of leaf-like processes projecting from the primary blade at a right angle (see Enation). Sometimes these are developed in a tubular form, so as to form a series of little horn-like tubes, or shallow troughs, as in _Aristolochia sipho_. At other times the nerves or ribs of the leaf project beyond the blade, and bear at their extremities structures similar to those just described. [Illustration: FIG. 167.--Lettuce leaf, bearing on the back a stalked cup, arising from the dilatation of the stalk (?).] In a variety of _Codiaeum variegatum_ a similar formation may be seen to a minor extent. Even the common _Scolopendrium vulgare_ occasionally produces small pitchers of this character, as in the varieties named _perafero-corautum_, Moore, and _peraferum_, Woll.[350] In carnations leaves may sometimes be seen from both surfaces, from which project long, sharp-pointed tubular spurs at irregular intervals. A very singular illustration of this is figured by Trattinick,[351], in which the leaves, epicalyx, sepals, and petals, were all provided with tubular spurs.