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A TEXTBOOK OF THEORETICAL BOTANY 



into a tight ball, aided therein by the sticky nature of the pollen itself, and 

 by the use of its elongated maxillary palp, which can be rolled up to hold 

 the pollen-ball. Having collected all the pollen it can carry, the moth 

 departs to another flower, where it alights on the stamen filaments, pierces 

 the ovary wall with its ovipositor and lays a batch of eggs alongside the 

 ovules of the plant. Lastly it crawls up the style and pushes its ball of 

 pollen firmly into the cavity between the lobes of the stigma. 



The moth eggs hatch in four or five days and the grubs eat the Yucca 

 ovules, each of them eating some twenty ovules. When fully developed they 

 bite their way out of the ovary and lower themselves to the ground, where 

 they pupate until the next flowering season of the Yucca. 



Only about 20 per cent, of the ovules are destroyed, while the rest 

 develop into seeds, and as no seeds are formed in the absence of the 

 moth, it is plain that the survival of the genus Yucca depends on the opera- 

 tions of Pronuba. Likewise, the grubs are ensured a supply of food from 

 the developing ovules by the act of pollination, without which Pronuba 

 yuccaseUa would also perish. Apparently nearly all the species of Yucca are 

 in a similar position of dependency, for only in one species has a transfer- 

 ence of pollen in any other way been observed to occur. 



A similar though much less striking case of mutual dependence is that 

 of Sileiie nutans (Fig. 1208), which is typical of a number of other Caryo- 

 phyllaceae and some Leguminosae and Rosaceae. This plant is a night- 

 bloomer, the flowers being closed all day and showing only the brownish 

 outer sides of the sepals and petals. At dusk its flowers open and display a 



Fig. 1208. — Silene nutans. A, Night condition. B, 

 albimacula is shown pollinating a night Hower. 

 glands on the stem. {After Kerner.) 



Day condition. The moth Dianthoecia 

 Notice also an ant caught by the sticky 



