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The more distant the lobes, the deeper the ascidium will become. It should be added that this explanation of the different shapes of ascidia is of general validity.
Of the lime-tree, instances are known of single trees which produce hundreds of such anomalous leaves yearly, and one such a tree is growing in the neighborhood of Amsterdam at Lage Vuursche. I have alluded to these cases more than once, but on this occasion a closer inspection of the structure of the ascidium is required. For this purpose we may take the lime-tree as an example.
If the leaves are sessile, the analogy with the teasels is complete, as shown, for instance, in a case of Cotyledon, a crassulaceous plant which is known to produce such cups from time to time. They are narrower than those of the teasel, but this depends, as we have seen for the "one-leaved" ascidium, on the shape of the original leaf.
The base of this apical limb exhibits two connate lobes, forming together a wide cup or ascidium. It should be stated that these interruptum varieties are highly variable, especially in the relative size of the three principal parts of the leaf.
Numerous curious contrivances in these structures to catch ants and other insects have been described, but as they have no relation to our present discussion, we shall abstain from dealing with them. Likewise we must refrain from a consideration of the physiologic qualities of the tendril, and confine our attention to the combination of a limb, a naked midvein and an ascidium.
This connation of the basal lobes is universally considered as a good and normal specific character. Nevertheless it has its manifest analogy in the realm of the anomalies. This is the pitcher or ascidium. It is probable that both these forms have varieties with, and others without, ascidia.
Those of the Croton, according to our knowledge regarding similar cases, must have arisen at once, and hence the conclusion that the ascidia of Nepenthes are also originally due to a sudden mutation. Interrupted leaves, with an ascidium on a naked prolongation of the midvein, are by no means limited to the Croton varieties.
In the case of stalked leaves the two opposite stalks must, of course, constitute a long and very narrow tube, when growing together. This tube must bear at its summit the conical ascidium produced by the two connate limbs. At its base however, it includes the terminal bud of the stem, and frequently the tube is so narrow as to impede its further development.
Though it is of course conceded that the ascidium of Nepenthes has many secondary devices which are lacking in Croton, it seems hardly allowable to deny the possibility of an analogous origin for both.
By this contrivance the double ascidium assumes a terminal position. Instances have been observed on magnolia, in Boehmeria and in other cases. Flowers on leaves are of rare occurrence. Notwithstanding this, they constitute specific characters in some instances, accidental anomalies in others. Helwingia rusciflora flora is the most curious and best known instance.
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