369 



shite about a week before the Yuecas blo(niv again, and finally issues 

 as a uiotli to continue the annual cycle ot its career. The chrysalis 

 (Fig. 67) has a capitate s[)ine and a series of dorsal si)ines, some of 

 which are spatulate and admirably fitted for l»el])in.i; it to work thrcmgh 

 tiie ground. 



The effect of the puncture of the female moth on tiie fruit is at 

 once noticeable by a darker green discoloratiou externally. In time 

 this be(;omes a depression, causing a constri<'tion of the pod, and tlie 

 irregularities of the pod (Pig. 68, />, o), which have been supposed to 

 be characteristic of the genus Yucca, are really due to these ])unctures, 

 which ordinarily occur just below the middle. The absolute need of 



Fig. 08. — Matui-f pods of Yucca aiiguati/olia: a. artiticiiilly poUiuized and protected from Pronuba; 

 b, normal pod, allowing constrictions resulting from Pronuba puncture and exit holes of larva; c, one 

 of the lobes cut open, showing larva within. 



Pronuba in the pollination of our dehiscent Yuccas I have proved over 

 and over again in many ways. The i)laut never produces seed where 

 Pronuba does not exist; 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 insure 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 suci'essfully as it is done by Pronuba. It is Promiba yuccasella 

 which pollinizes all our Y^uccas east of the Rocky Mountains, so far as 

 known, and the species is remarkably uniform in character, its appear- 

 ance being coetaneous with the flowering of Yucca filamentosa. On the 

 western plains its appearance has become adapted to the flowering of 

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

 are frequently grown side by side, Y. (inffitsfifolia flowers two or three 

 weeks earlier than Y. filamentosa, and generally too earlj^ to receive the 

 visits of Pronuba, so that it produces seed only on very rare occasions. 



