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 stigma. On 

 the stigma the pollen falls. In some 

 plants these organs are separated in 

 diSferent flowers or even on different 

 plants. 



The flowers of insect-pollinated 

 plants, on the other hand, are usually 

 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 easily 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 anothe 



notably 



IS of 



: iin flowers which 



early pi 



plished. Many flowt 

 chids, have special in 

 ture apparenth di v 1 

 of securing cross j 

 and preventing 

 bodies of some in 

 sponding adapt iti ii 

 cross-pollination of i 

 they are in the habit of visiting most tre 

 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 1886. For the distinction between fer- 

 tilization and polIinMtion, see the article 



The vcilii.- Mf ,r..sM,,- to plants was first 

 ■s Darwin in 1859. 

 lis 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 

 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, »874.^ ?,',!;i';'."l! '^JuLT^"^"' 

 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 fiower, which favor 

 cross-pollination. (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 and the i>n.ji. tiim ,iyl.. 



more common than the prematurity of the 3- Buttti |., viMnvinu-iii.iny 



stamens. (3) Self-sterility, which is the small pistils n, tl„. .fni.r ,ui.l 

 inability of a flower to set fruit with its ^'I'^oUom -ph\TO sl'ow'in» 

 own pollen. It might be expected that the Sparted stigma.' and thS 

 self-stenhty would naturally result from stamens included in the tube. 



POLLINATION 





,-pollii 



eloped in 



lUustrate Pollination. 



1. Top.— The stnictur 

 plum blossom: se. sepn 

 petals: s(a. stamens: " < 

 s, style: st. stigma. Tin' 

 is composed of tlio < 

 style and stigma. It c<n 

 the female part. The st: 

 are tipped with aulht 

 which the polle 

 part, is borne. 



2. Fuchsi:i,«h 



Thee 



evidence tli:it tin- 

 noticeable ill ]il:i]il- 

 this way. Srlf.^t. 

 ally due to ;i .Uticifiicv in the pollen 

 or to defective pistils. The pollen 

 grains often gerniiiKile on the stigma, 

 but fertilizatiou 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, (i) The separa- 

 tion of the sexes in different flowers or on 

 different individuals. It is thought by 

 some that there is a gradual evolution 

 among sonic kinds of plants toward uni- 

 sexuality. and that adaptations for insect- 

 pollination, dichogamy and self - sterility 

 are steps in this process. 



Self-sterility is cninmon 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 afliuity between the two. 

 It is not a constant factor in any variety, 

 but s.-inis tube as easily influenced by the 

 is.nthtions under which the tree is grown 

 as Is tin- ,1/c . shape or color of the fruit. 



mate 111- I' K '.,,],, i li It --. If sterility. 

 Ther.tM- t sterile in 



iiade 



another vancfv. Tims, .Mm, i 1" .n- ti. . ly 

 if planted with De Soto, ;,ih1 Ki.-rt.r «ith 

 Le C'onte. 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 

 affinity between the two, so that the pistils 

 of the self-sterile variety will accept the 

 pollen of the other and develop into good 

 fruit. Stich afliuity 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 v:iriiiics 

 wlii'Ii are inclined to In- nnituiTlul \^llon 

 I>I:iiiT,'d alone, and disro\'rin^' \\li:it :tre 

 ih. l.est pollinizers for ili.ni. E\|iiri- 



or<-ii:ir-ls lt:ive indicated that nearly all 

 v:iri. ti.s of orch.ard fruits, whether self- 

 st.rilo ..r not. will produce better fruit 

 with |."ll. II ,,iher than their own. The 

 l'.:iM\\ ill 11 1, pi r will usually bear good fruit 

 if |il:niio,i :iioiie, but it will bear better 

 fiiiii 11 iho liirlif variety is planted with it. 

 Till- iirol.iiliility is that most of our leading 

 commercial varieties commonly planted in 

 large blocks will produce enough better 

 fruit by a judicious intermingling of ona 



