POLLEN 



POLLEN 



2735 



or both. The pollen is more or less 

 moist or sticky, so that it is not 

 easily blown away. Insects are prob- 

 ably attracted by the showy colors 

 and by the perfume, both of which 

 bespeak the presence of nectar. As 

 the insect reaches down for the 

 nectar, which is near the bottom of 

 the flower, some parts of its body are 

 almost sure to become dusted with 

 pollen. When the insect visits another 

 flower some of this pollen may be 

 brushed upon the stigma and a fresh 

 supply received. This pollen likewise may 

 be carried to another flower, and so on. 

 Thus cross-pollination, or the transfer of 

 pollen from the anthers of one flower to 

 the pistil of another, is accomplished. 



Many flowers, notably the orchids, have 

 special modifications of structure appar- 

 ently developed for the purpose of securing 

 cross-pollination by insects and pre- 

 venting self-pollination. The bodies of 

 some insects, also, have corresponding 

 adaptations which insure the cross-pollina- 

 tion of certain flowers which they are in 

 the habit of visiting most frequently. This 

 correlation between flowers and their insect 

 visitors has been the subject of extended 

 observation. ''Fertilization of Flowers/' 

 by Herman Muller, contains a bibliog- 

 raphy of the subject up to 1886. For the 

 distinction between fertilization and pol- 

 lination, see the article Fertilization, page 

 1221. 



The value of crossing to plants was first 

 clearly proved by Charles Darwin in 

 1859.* From the observations of Kolreuter, 

 Sprengel, Knight, and his own exhaustive 

 experiments, Darwin showed that con- 

 tinued self-fertilization is likely to result 

 in inferior offspring; while cross-fertiliza- 

 tion, within certain limits, gives greater 

 vigor to the offspring. Cross-fertilization 

 between different flowers on the same plant 

 usually has no appreciable advantage. 

 The probable reason for this is that the 

 plant resulting from the union of unlike 

 parents, as in cross-fertilization between 

 flowers on different plants, is more variable 

 than one resulting from self-fertilization 

 or crossing between different flowers on 

 the same plant, and hence has a better 

 chance of fitting into new conditions. 



Plants are endlessly modified to secure 

 cross-fertilization and avoid self-fertiliza- 

 tion. The principal means by which this 

 end is gained are: (1) Special contrivan- 

 ces in the structure of the flower which 

 favor cross-pollination. (2) A difference 

 in the time at which the pollen matures 

 and the stigmas become receptive in the 

 same flower (dichogamy). This condi- 

 tion is very noticeable in some varieties 

 of orchard fruits. The prematurity of 

 the pistil is more common than the 

 prematurity of the stamens. (3) Self- 

 sterility, which is the inability of a 

 flower to set fruit with its own pollen. 

 Self-sterility is not usually due to a 

 deficiency of pollen or to defective pistils. 

 The pollen-grains often germinate on the 

 stigma, but fertilization does not take 

 place. The embryological reasons for 

 this are not clearly understood. The ulti- 

 mate cause of self-sterility in the grape 



tit ~ 



Sta 



3098. Structure of the flower, 

 to illustrate pollination. 



1. Top. The structure of a 

 plum blossom: se, sepals; p, 

 petals; to, stamens; o, ovary; 

 *, style; st, stigma. The pistil 

 is composed of the ovary, 

 style, and stigma. It contains 

 the female part. The stamens 

 are tipped with anthers in 

 which the pollen, or male part, 

 is borne. The ovary, o, ripens 

 into the fruit. 



2. Fuchsia, showing ovary at 

 a, 3 stamens (one is removed) 

 and the projecting style. 



3. Buttercup, showing many 

 small pistils in the center and 

 stamens surrounding them. 



4. Bottom. Phlox, showing 

 the 3-parted stigma, and the 

 stamens included in the tube. 



has been studied by Dorsey. Cyto- 

 logical studies of the pollen of self- 

 sterile varieties showed distinct de- 

 generative processes in the generative 

 nucleus, or arrested development pre- 

 vious to mitosis in the microspore 

 nucleus. Dorsey concludes that self- 

 sterility in the grape is not due to 

 hybridity alone, as suggested by 

 Beach, since there are both fertile 

 and sterile hybrid varieties; but is 

 due also to deep-seated influences 

 operating to produce declinism and dioeci- 

 ousness, the native species of grapes 

 being mostly dioecious. Dorsey finds the 

 nuclei of the pollen of many self-sterile 

 varieties of native plums to be degenerated 

 and disorganized. Degeneration of the 

 pollen cannot be the main cause of self- 

 sterility, however, since two self-sterile 

 varieties may be mutually fruitful when 

 planted together. About sixty species 

 of plants are known to be more or less 

 self-sterile. (4) The separation of the 

 sexes in different flowers or on different 

 individuals. It is thought by some that 

 there is a gradual evolution among some 

 kinds of plants toward unisexuality, and 

 that adaptations for insect-pollination, 

 dichogamy, and self-sterility are steps in 

 this process. 



Self-sterility has an important economic 

 aspect in the culture of certain fruits. It 

 is common in varieties of pears, apples, 

 plums, and grapes; it is uncommon or 

 unknown in cherries, peaches, raspberries, 

 currants, gooseberries, and strawberries. 

 Whenever isolated trees or large blocks of 

 a variety blossom full year after year, but 

 drop most of the fruit before it is half- 

 grown, the variety may be self-sterile, 

 provided the failure cannot be attributed 

 to excessive vegetative vigor, marked lack 

 of vigor, disease (especially scab, brown- 

 rot, and fire blight), insect attack, unfavor- 

 able weather during the blossoming sea- 

 son, or other untoward circumstance. Self- 

 sterile varieties are detected experiment- 

 ally by inclosing the unopened blossoms 

 in thin paper sacks, and dusting the 

 pistils, when receptive, with the pollen 

 produced by these blossoms ; or by emas- 

 culating them and hand -crossing with 

 pollen of the same variety. If very few 

 fruits are produced from a large number 

 of these selfed blossoms, but the variety 

 fruits abundantly when crossed with other 

 sorts, it is self-sterile. A few varieties of 

 fruits are more or less self-fruitful, as 

 distinct from self -sterile; they bear good 

 fruit with their own pollen, but the fruits 

 are seedless, as in the banana. Ewert 

 found that many apples in Germany 

 have this parthenocarpic development; 

 that is, they grow without fertilization. 

 It is not common in North American 

 varieties of fruits. 



Self -sterility is not a constant factor in 

 any variety. It appears to be almost as 

 easily influenced by the conditions under 

 which the plant is grown as is the shape 

 or color of the fruit. A variety is fre- 

 quently self-sterile in one locality and 

 self-fertile in another. Waite found sev- 

 eral varieties of Japanese plums self- 

 sterile, but concluded, "With plums, as 



