392 ANGIOSPERMAE—DICOTYLEDONES 



933. Pyrus communis L. (Herm. Muller, 'Fertilisation,' p. 239, 'Weil. 

 Beob.,' p. 234; Kirchner, ' Beitrage,' pp. 35-6; Swayne, Trans. Hort. Soc, London, 

 V, 1 88 1, p. 208; Waite, 'The Pollination of Pear Fls.,' 'Pollination of Pomaceous 

 Fruits,' 'The Fertilisation of Pear Fls.'; Knuth, ' Bloemenbiol. Bijdragen,' ' Weit. 

 Beob. ii. Bl. u. Insekt. a. d. nordfr. Ins.,' p. 234.) — Hermann Miiller was the first 

 to call attention to the pronounced protogyny of the flowers of this species, and 

 also briefly described their mechanism. Kirchner has given the following exhaustive 

 account of the same. The anthesis of individual flowers lasts from seven to eight 

 days. They possess an odour like that of hawthorn (due to trimethylamide), or, 

 as Kirchner puts it, smell like cockchafers. There are many differences in size 

 and form, according to the variety. Sometimes the flowers are bell-shaped, the 

 petals inclining obliquely upwards; the average diameter in such cases is 18 mm. 

 In other varieties the corolla spreads out flat, so that its diameter attains 42-48 mm. 

 When the flowers open the central styles, with their already receptive stigmas, 

 stand tolerably erect : all the stamens are so bent inwards that their red unripe 

 anthers are crowded together in a heap in the middle, a little below the level 

 of the sligmas. They guard the passage to the nectar. It follows that insect 

 visitors usually alight upon the stigmas, and effect crossing if they have previously 

 visited an older flower. This female stage lasts from two to four days, according 

 to the weather. Meanwhile the outermost stamens gradually erect themselves, 

 diverge obliquely outwards, and open their anthers. The inner stamens in turn 

 behave in the same way, till in from five to seven days all the anthers have dehisced. 

 The petals have sometimes been shed before the anthers of the innermost five 

 stamens have opened. When the flower fades the stamens retain their divergent 

 position, but the styles curve away from one another so as to bring the stigmas 

 into contact with the shrivelled anthers. Since some pollen still clings to these, 

 automatic self-pollination is effected. The flowers are not protected against rain, 

 but are not very sensitive to its influence. 



Cross-pollination is absolutely essential. This was first proved by George 

 Swayne, and confirmed by Rlerton B. Waite. The latter states that, as a rule, 

 ]ierfect fruits only result from crossing, while pollen from another tree of the same 

 variety has no more effect than that of the same flower. 



Waite drew the following conclusions from his pollination experiments. — 



1. Many of the common varieties of pears require to be crossed, and produce 

 imperfect fruits or none at all when fertilized with their own pollen. 



2. Some varieties are fertile with their own pollen. 



3. In effecting crossing it is not sufficient to use pollen from another tree 

 of the same variety, for this is not more effective than the pollen of the same 

 tree. It is necessary to make use of pollen from another variety. 



4. This ineffectiveness of pollen is not absolute, but is due to want of affinity 

 between the pollen and ovules of the same variety. 



5. It follows that pollen taken from two varieties may be perfectly ineffective 

 when transferred, respectively, to the stigmas of flowers of the same variety, but 

 each may produce excellent results when applied to stigmas of the other variety. 



II. Pears resulting from self-fertilization contain imperfectly formed seeds, 

 that are usually quite vestigial. The fruits that result from crossing contain well- 

 developed healthy seeds. 



