REPRODUCTION AND DISPERSAL 



853 



a 



and of the shedding of pollen. Stigmas commonly remain receptive 



(especially when unpollinated) for a longer time than that required for 



the shedding of the pollen, hence cross pollination 



is more likely to result when flowers are protan- 



drous (i.e. with the anthers maturing first) than 



when they are protogynous (i.e. with the stigmas 



maturing first); however, pollination of some 



kind, either cross or close, is more likely to result 



when the flowers are protogynous, because of 



the greater likelihood of overlap in the latter case. 



Probably the number of protandrous and protogynous 

 species is about equal, though there are a greater number 

 of conspicuously protandrous forms, such as the saxifrage 

 (fig. 1177), the evening primrose 

 (in which the anthers may shed 

 before the corolla opens), the 

 composites, and the umbelli- 

 fcrs, than there are of conspic- 

 uously protogynous forms, such 

 as the figwort (figs. 1178, 1179) 

 and the crucifers. A striking 

 case of protogyny occurs in 

 Aristolochia Clemalitis, where 

 the interior of the narrow calyx 

 tube is lined with reflexed hairs. 

 Insects enter easily and crawl 

 over the mature stigma, but on 

 account of the stiff hairs they 

 cannot leave until the anthers 



mature, when they become dusted with pollen; the subse- 

 quent withering of the calyx hairs permits their exit, and 

 upon their entering another flower, cross pollination takes 

 place. 



In most dichogamous flowers the stigmas and the an- 

 thers, though usually occupying the same position consecu- 

 tively, nevertheless are out of the way of the one, when the 

 other is mature. In the mallows the anthers at first hide 

 the stigmas, but later bend back and expose them, while 

 in Salvia the style which at first is short grows out after 

 the pollen is shed, assuming a favorable position for pollen reception by the stigma. 

 In the figwort, which has a protogynous flower, the style bends back over the lip 

 after maturity (fig. 1179). In Parnassia one stamen after another assumes a posi- 

 tion where visiting insects are likely to come into contact with them. In most 

 dichogamous flowers (but not in Aristolochia) two insect visits are necessary if both 



FIGS. 1178, 1179. 

 Flowers of the figwort 

 (Scrophularia marilast- 

 dica), illustrating pro- 

 togyny: 1178, a young 

 flower with a promi- 

 nently exserted style (t) 

 and a receptive stig- 

 matic surface (g); 1179, 

 the same flower a day or 

 two later, with its style 

 (0 declined and out of 

 the way of visiting in- 

 sects, the stamens hav- 

 ing grown sufficiently to 

 expose the anthers (a) 

 to such pollinating 

 agents; note that the 

 sympetalous corolla (</) 

 is bilabiate ; c, calyx. 



FIG. 1177. Flowers 

 of a saxifrage (Saxi- 

 fraga sarmentosd), illus- 

 trating protandry; in 

 the younger (upper) 

 flower, the anthers (a) 

 are mature and the pi> 

 tils (5) immature; in the 

 older (lower) flower, the 

 anthers (a') have shed 

 their pollen and the pis- 

 tils (5') have become 

 mature; note that the 

 corolla is zygomorphic, 

 three of the petals (p) 

 being short and two 

 long (/>') 



