Page:Popular Science Monthly Volume 41.djvu/192

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180
THE POPULAR SCIENCE MONTHLY.

similarly difficult and complicated oviposition of the Cynipidæ, as Adler has shown. In dissecting the young fruits of the filamentose yuccas, with a view to critical examination, I have found that about half of them, on the average, contain nothing; but the proportion varies greatly in different localities and according to circumstances, and I may say that, as a result of my numerous examinations, fully two thirds of the mature pods are found to contain the larvae of Pronuba. All the experiments which I have so far made, or have known to be made, prove conclusively that the capsular species never set fruit without her aid.

Pronuba yuccasella is found in all parts of the country east of the Rocky Mountains where the filamentose yuccas normally range; but has not extended to all sections where they are cultivated. The time of its appearance is strikingly coetaneous, east

PSM V41 D192 Mature pods of yucca angustifolia.jpg

Fig. 10.—Mature Pods of Yucca angustifolia: a, artificially pollinized and protected from Pronuba; b, normal pod, showing constrictions resulting from Pronuba puncture and exit-holes of larva; c, one of the lobes cut open, showing larva within.

of the Mississippi, with the blooming of filamentosa; while other cultivated species which bloom either earlier or later, and which, therefore, do not receive the visits of the moth, I have, as already stated, never known to bear seed. On the Western plains, where Y. angustifolia is native, the moth's season of appearance is adapted to the flowering of this particular yucca. In California, Yucca whipplei is pollinized by Pronuba maculata, an invariably maculate species; while, on the Mojave Desert, Yucca brevifolia is pollinized by Pronuba synthetica, a species still more abnormal than yuccasella and modified to fit it to the peculiarities of that particular species of yucca. In the Gulf States the typical yuccasella occurs, and fertilizes not only the filamentose yuccas, but