94 



Riley Some Interrelation* of Plant* and 7//,srr/x. 



ways. The plant never produces seed where Pronuba does not 

 e xist ; it never produces seed when she is excluded artificially, 

 and experiments which I have made with artificial or brush 

 pollination all show that it is much more difficult to ensure 

 complete fructification than would at first appear, and that the 

 act of pollination is rarely performed with a brush or by using 

 the flower's own filaments, as successfully as it is done by Pro- 

 nuba. It is Pronuba yuccasella which pollinizes all our Yuccas 

 east of the Rocky Mountains, so far as known, and the species 

 is remarkably uniform in character, its appearance in time being 

 coetaneous with the flowering of Yucca filamentosa. On the. 

 western plains its appearance has become adapted to the flower- 



Fi<;. !). Mature pods of Yucca anyustifoda : a, artificially pollinized and protected 

 from Pronuba; 6, normal pod, showing constrictions resulting from Pronuba puncture 

 and exit holes of larva ; c, one of the lobes cut open, showing larva within. 



ing of Yucca angustifolia, but in the east, where these two species 

 of Yucca are frequently grown side by side, Y. anausttfolia flowers 

 two or three weeks earlier than Y. filamentosa and generally too 

 early to receive the visits of Pronuba, so that it produces seed 

 only on very rare occasions. Yucca brevifolia is pollinized by 

 Promt h xt/,ifketica Riley, the most remarkable species of the 

 genus, having very stout maxillary tentacles, a very stout ovi- 

 positor, shorter than that of yuccasella, but characterized chiefly 

 by having fuliginous and unsealed wings and a polished, naked 

 and flattened body structures all well adapted for crawling be- 

 tween and about the compact and crowded flowers, with their 



