POLLINATION 



flower, — the stamens and pistils (see 

 Flower; also Fig. 1874). The sta- 

 mens bear the pollen in their anthers, 

 and they die after the pollen is shed. 

 The pistils bear the ovary or seed- 

 case, the style, and the stiy;nui. On 

 the stigma the pollen falls. In some 

 plants these organs are si'parated in 

 different flowers or even on different 

 plants. 



The flowers of insect-pollinated 

 plants, on the other hand, are nsually 

 characterized by being showy and having 

 nectar or fragrance, or both. The pollen 

 is more or less moist or sticky, so that it 

 is not easilj' blown away. An insect is 

 attracted to these flowers by the showy 

 colors and the perfume, two things 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 may 

 likewise be carried to another flower, and so 

 on. Thus "cross-pollination," or the trans- 

 fer of pollen from the anthers of one 

 flower to the pistil of another, is accom- 

 plished. Many flowers, notably the or- 

 chids, have special modifications of struc- 

 ture apparently developed for the purpose 

 of securing cross-pollination by insects, 

 and preventing self-pollination. The 

 bodies of some insects also have corre- 

 sponding adaptations which insure the 

 cross-pollination of certain flowers which 

 they are in the habit of visiting most fre- 

 quently. This correlation between flowers 

 and their insect visitors has been the sub- 

 ject of extended observation. "Fertiliza- 

 tion of Flowers," by Hermann Miiller, 

 contains a bibliography of the subject up 

 to 188(5. For the distinction between fer- 

 tilization and pollination, see the article 

 Fertilization, page 579. 



The value of crossing to plants was first 

 clearly proved by Charles Darwin in 1859. 

 Prom 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-fertili- 

 zation, within certain limits, gives greater 

 vigor to the offspring. Cross-fertilization 

 between different flowers on the same 

 plant has usually no appreciable advan- 

 tage. The reason for this is that the 

 plant resulting from the union of two 

 unlike parents, as in cross-fertilization 

 between flowers on different plants, is 

 more variable than the plant resulting 

 from self-fertilization or crossing be- 

 tween different flowers on the same plant, 

 and hence has better chance of fitting 

 itself to new conditions. Plants are end- 

 lessly modified to secure cross - fertili- 

 zation and avoid self-fertilization. The 

 principal means by which this end is 

 gained are: (1) Special contrivances in 

 the structure of the flower, which favor 

 jross-poUination. (2) A difference in the 

 time at which the pollen matures and the 

 stigma becomes receptive in the same flow- 

 er (dichogamy). This condition 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. It might be expected that 

 self-sterility would naturally result from 



POLLINATION 



1389 



1874. Structure of the Flower, 

 to illustrate Pollination. 



1. Top.— The structure of a 

 plum blossom: se. sepals; p. 

 petals; std. stamens; o ovary; 

 s. 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. 



continued cross-pollination by the 

 first two means, hut tliiTc is little 

 evidence that the self-sterility now 

 noticeable in plants was developed in 

 this way. Self-sterility is not usu- 

 ally due to a deficiency in the pollen 

 or to defective pistils. The pollen 

 grains often germinate on the stigma, 

 init fertilization does not take place. 

 The embryological reasons for this 

 are not clearly understood. About 

 sixty species of plants are known to be 

 more or less self -sterile. (4) The separa- 

 tion of the sexes in different flowers or on 

 different individuals. It is tliought by 

 some that there is a gradual evolution 

 among some kinds of plants toward uni- 

 sexuality, and that adaptations for insect- 

 pollination, dichogamy and self - sterility 

 are steps in this process. 



Self-sterility is common in varieties of 

 orchard fruits, particularly in pears and 

 plums, and in grapes. Whenever isolated 

 trees or large blocks of a single variety 

 blossom full, year after year, but drop 

 most of the fruit before it is half-grown, 

 the trees may be self - sterile, provided 

 the failure cannot be attributed to fungous 

 disease, insect attack, frost or other in- 

 jury. Familiar examples of self-sterile 

 varieties are : Wild Goose and Miner 

 plums, Kieffer and Bartlett pears and 

 Esopus Spitzenburg apple. Self-sterility 

 in orchard fruits does not usually result 

 from defectiveness of pollen or pistil, but 

 from a lack of affinity between the two. 

 It is not a constant factor in any variety, 

 but seems to be as easily influenced by the 

 conditions under which the tree is grown 

 as is the size, shape or color of the fruit. 

 The adaptation of a variety to soil and cli- 

 mate has much to do with its self-sterility. 

 Therefore, a variety is often self-sterile in 

 one place and self-fertile in another. 



A self-sterile tree often may be made 

 fruitful by planting near it trees of an- 

 other variety to supply pollen, or by top- 

 grafting part of the tree with cions of 

 another variety. Thus, Miner bears freely 

 if planted with De Soto, and Kieffer with 

 Le Conte. No benefit is derived from 

 planting in an orchard of one variety new 

 trees of the same variety. There are two 

 important points to be considered in the 

 choice of a pollinizer. First, the two 

 varieties must blossom together, so that 

 cross-pollination by wind or insects may 

 take place. Second, there must be an 

 atfinity between the two, so that the pistils 

 of the self-sterile variety will accept the 

 pollen of the other and develop into good 

 fruit. Such affinity can be determined only 

 by experiment. In a large orchard of a 

 self-sterile variety which is valuable for 

 market, every third or fourth row should 

 be the pollinizer. 



Orchard Pollination, however, is a larger 

 problem than the detection of varieties 

 which are inclined to be unfruitful when 

 planted alone, and discovering what are 

 the best pollinizers for them. Experi- 

 ments in crossing and observations in 

 orchards have indicated that nearly all 

 varieties of orchard fruits, whether self- 

 sterile or not, will produce better fruit 

 with pollen other than their own. The 

 Baldwin apple will usually bear good fruit 

 if planted alone, but it will bear better 

 fruit if the right variety is planted with it. 

 The probability is that most of our leading 

 commercial varieties commonly planted in 

 large blocks will produce enough better 

 fruit by a judicious intermingling of on« 



