854 



HORTICULTURE 



December 28, 1907 



SOME PHASES OF POLLINATION. 

 Results with Sacked Blossoms. 



The question o£ the potency oJ pol- 

 len as a factor in the fertilization of 

 the blossoms of fruits is comparatively- 

 new. Some years ago the writer 

 tested the pollen of various varieties 

 of grapes that seemed to show that 

 the viability of the pollen was the 

 chief it not the only factor which de- 

 termined whether a given variety was 

 self-fertile or self-sterile. This natur- 

 ally suggested the query as to whether 

 the same conditions did not possibly 

 exist among other of our cultivated 

 fruits. In apples, in particular, as the 

 most important of our fruits, an inves- 

 tigation was started with a view to 

 determining the self-fertilizing capa- 

 city of the various varieties. This 

 worli has been continued now for 

 about four years in a very desultory 

 way, owing to interruptions caused by 

 the intervention of other work, change 

 of location, etc. 



One of the first, and almost indis- 

 pensable things in an investigation of 

 this kind, is a complete and accurate 

 list of the self-fertile and self-sterile 

 sorts. Unfortunately, there are no 

 such lists which are generally ac- 

 cepted. Many investigators question 

 the accuracy of the results secured by 

 sacking the blossoms of such fruits as 

 apples and pears, basing their skepti- 

 cism on the fact that many varieties 

 which fail to set fruit in sacks do not 

 fall to give satisfactory crops when 

 planted under conditions where access 

 of pollen from other varieties would be 

 improbable, if not impossible! Several 

 experimenters, to my knowledge, have 

 sacked large numbers of blossoms for 

 several years in succession with such 

 contradictory results that they were 

 never published. Generally the results 

 of such experiments prove altogether 

 too much. Varieties of apples which 

 are recognized to be self-fertile will 

 frequently set no fruit at all under 

 sacks, and will almost invariably set 

 much less than when exposed to the 

 visiting insects; and this is true even 

 when the trees are standing in solid 

 blocks of one variety so that cross pol- 

 lination of the outside blossoms Is 

 very improbable. 



Confusing Results witli Apples. 



In some experiments carried on by 

 Lowe and Parrott, and kindly loaned 

 me by Mr. Parrott for this occasion, a 

 small Ben Davis tree was completely 

 covered with muslin. On this tree 

 there were 315 blossoms but only five 

 set fruit. Other trees of the same va- 

 riety alongside set the usual crop. 

 Yet Ben Davis is known to be a self- 

 fertile variety. Such instances could 

 be duplicated many times from the ex- 

 perience of every horticulturist who 

 has investigated this subject to any 

 extent. On the Geneva Station this 

 year sacks were placed on the blossoms 

 of 60 varieties, from ten to twenty 

 sacks being placed on each tree. Tol- 

 man Sweet, which is sometimes 

 classed as a self-sterile sort, set seven 

 fruits in one sack. Oldenburg, which 

 is known to be self-sterile, gave no 

 fruits whatever, although the twigs 

 surrounding the sacks were covered 

 with the usual crop. The results of 

 other varieties were equally confusing. 

 Such experiments as this are not worth 

 publishing; all that they indicate is 



that sacking alone will not determine 

 for the apple, as it does for the grape, 

 whether the variety is self-fertile or 

 self-sterile. The question naturally 

 arises why is it that a variety normally 

 self-sterile should fail to give fruit un- 

 der sacks. It manifestly must be on 

 account of some abnormal condition 

 produced by the presence of the sack. 

 A manila sack being close and imper- 

 vious, will naturally shut out more or 

 less of both light and air, but the same 

 is not true of muslin sacks, yet they 

 give the same contradictory results as 

 the paper ones. In Lowe and Parrott's 

 experiments, where they covered small 

 trees or large branches with muslin, 

 windows were made of wire screens for 

 the admission of light and air. In no 

 case did they find that there was any 

 more fruit close to the windows than 

 away from them, and there were no 

 more fruits in the muslin sacks with 

 windows than in those without. So it 

 seems that it can hardly be a question 

 of the lack of light and air. In put- 

 ting a sack over a cluster or several 

 clusters of blossoms it is very difficult 

 not to have the sides of the sack so 

 close to the blossoms that the natural 

 movement of the sack in wind would 

 cause it to press against certain of the 

 stigmatic surfaces after the petals have 

 expanded. It seemed a question as to 

 whether this possible injury might pre- 

 vent fertilization from taking place. 

 Examination of a large number of 

 sacked blossoms, however, fails to show 

 that there was any greater probability 

 of the fruit that set being in the center 

 and more protected than in the outer 

 portion of the sack. This year four 

 fruits were found, each in different 

 sacks, where the sack had been pulled 

 down on the branch so far that the 

 blossom was bent back upon itself so 

 that the calyx end of the fruit pointed 

 down upon the branch upon which it 

 was borne. Yet fruit had set under 

 these conditions. In numerous other 

 cases fruit had set with the calyx 

 liressins against the sides of the sack, 

 bhowin.j; that the contiquity of the 

 sack apparently had no deterrent ef- 

 fect on the setting of fruit. There 

 seems to be only one other abnormal 

 condition produced by the sacks, and 

 this is the prevention of insect visits. 

 The use of the sacking method as a 

 means of determining a fruit's capacity 

 for self-fertilization is based on the 

 supposition that it will prevent 

 insects from bringing pollen of 

 other varieties while the pollen 

 of the blossoms enclosed will 

 fall on their own stigmas. So far as 

 I am aware, it lias never been proved 

 that this is actually the case. The 

 stigma of the apple is receptive, even 

 before the blossoms open. If blos- 

 soms are castrated the day before 

 they normally open and pollinated at 

 the same time, they will usually set 

 fruit. How long the stigmas remain 

 receptive I do not know, but I have 

 never found as good results from the 

 later pollinations as from the earlier 

 ones; apparently showing that the 

 receptive condition is not long con- 

 tinued. When the blossom opens the 

 anthers usually have not burst, and 

 no pollen is available from that blos- 

 som. They soon commence bursting 

 from the outside in, that is, those at 

 the periphery dehisce first, and then 

 those toward the center. Thus it 



will be seen that the chance of the 

 stigma being past the receptive con- 

 dition before the closest stamens 

 would have available pollen is very 

 great, and that pollen from other and 

 earlier blossoms would be necessary 

 in order that fertilization should take 

 place. 



The Reason Possibly a Mechanical 

 One. 



A consideration of these facts makes 

 it appear possible that the reason why 

 apple blossoms seldom set fruit under 

 sacks as I'eadily as they do outside, 

 is a mechanical one and due to the 

 pollen not being on the legitimate 

 surface at the proper time. I have 

 been confirmed in this view by the 

 finding in one orchard this year that 

 in every case where fruit had set in 

 the sacks, the apple aphis was pres- 

 ent also. These sacks were wired to 

 the twigs, and while it might be pos- 

 sible that an aphis could crawl 

 through the opening, it does not seem 

 possible that it could do so with pol- 

 len sticking to its body without the 

 pollen being rubbed off. This coin- 

 cident was so marked that it was 

 possible to tell whether there were 

 apples in any particular sack by tear- 

 ing off the bottom of the sack and 

 looking for the cast skins of the 

 aphis. If these were present, then 

 the fruit could be confidently looked 

 for. In one case there were seven 

 fruits in one sack, the variety being 

 Tolman Sweet, which as is mentioned 

 above, is sometimes listed as being 

 self-sterile. For these reasons, and 

 others which the length of this paper 

 will not permit me to elaborate, I am 

 strongly of the opinion that we will 

 never secure satisfactory results from 

 sacking apples except where they be 

 also hand pollinated with their own 

 pollen. This manifestly is going to 

 add very seriously to the labor of all 

 such investigations. 



Extending the Blooming Season. 



One of the great diflficulties in doing 

 laboratory work with pollen is the 

 comparatively limited time during 

 which the varieties are in bloom and 

 pollen is consequently available. This 

 renders a proper verification of re- 

 sults impossible without extending 

 the work over several years. Work 

 which must be done in installments of 

 two or three weeks at a time, with a 

 year intervening between each period 

 of work, is necessarily unsatisfactory. 

 Even to the most enthusiastic there 

 is such a thing as the flagging of in- 

 terest in work so intermittent. For 

 these reasons I have tried to deter- 

 mine the feasibility of extending the 

 blooming season by forcing twigs of 

 the fruits in question into bloom in 

 the greenhouse during later winter 

 and early spring. Trial showed that 

 previous to January 1st twigs placed 

 under greenhouse copditions were 

 slow to start, and that some of the 

 buds would not start at all. It is 

 quite possible that by the use of 

 ether this difficulty could have been 

 overcome, but as it was considered 

 that a period of from four to five 

 months would be sufficient time for 

 the purpose, no effort was made to 

 extend the period beyond the date 

 mentioned. Some fruits can be forced 

 into bloom so that pollen can be 



