2 4 6 



NA TURE 



j>,. 15. 1885 



INVIG0RAT10N OF POTATOES BY 

 CROSS- BREEDING 

 C 1 M E interesting experiments on the potato were tried 

 ■^ at Reading last summer. Most persons are aware 

 that changes which are called "improvements" from a 

 commercial point of view have been effected among the 

 plants of the farm and garden in recent years by "hybri- 

 dising," and that the usual result of hybridising plants is 

 to invigorate them. Mr. Darwin explains the law which 

 horticulturists avail themselves of in the improvement of 

 plants when he says, " All forces throughout Nature tend 

 towards an equilibrium, and for the life of each organism 

 it is necessary that this tendency should be checked" 

 " Animals and Plants under Domestication," vol. ii. 

 p. 130). He adds, hence "the good effects of crossing 

 the bre d, for the germ will be thus slightly modified or 

 acted on by new forces." The invigoration consequent 

 on changing seed corn from one district to another is due 

 to the same causes, as well as the " evil effects of close 

 interbreeding prolonged during many generations, during 

 which the germ will be acted on by a male having almost 

 identically the same constitution." 



It would not be easy to ascertain the history of cross- 

 breeding in gardens. Hybridisation has been called " a 

 game of chance played between man and plants." All 

 the great breeders of florists' flowers, and of fruits and 

 vegetables, have practised the art successfully, but as 

 regards the potato recent investigations have shown that 

 the law of "changed conditions" has not been obeyed. 

 The term " hybridising," as used by horticulturists, is a 

 relative expression, referring sometimes to the crossing of 

 widely distinct forms, and in other cases to the injurious 

 union of closely connected forms. Hitherto the breeding 

 of potatoes has involved this vicious principle of too close 

 interbreeding, no other plant of the farm having been 

 more constantly intercrossed. 



Some years since the cross-breeding of English and 

 American potatoes was extensively practised, and to some 

 extent, undoubtedly, the " conditions of life" of the varie- 

 ties which were brought together from either side of the 

 Atlantic were changed ; but the cultivated potatoes both 

 of England and America belong to the same species, and 

 having both alike become enfeebled and subject to the 

 same disease, the experiment of interbreeding failed in its 

 object. 



Under these circumstances a 'veteran breeder wrote, 

 " I have come to the end of my tether ! " and he gave up 

 the breeding of potatoes in despair. This year he has 

 recommenced it, working hopefully with the aid of a 

 new species, and owing this new departure to the sugges 

 tion of the eminent botanist Mr. J. G. Baker, F.R.S., of 

 Kew. 



Mr. Baker undertook a scientific examination of the 

 various tuber-bearing species of Solanum, for the pur- 

 pose of ascertaining whether S. tuberosum, the cultivated 

 potato, might not possibly be invigorated by hybridising 

 it with some other species of the family. Writing in the 

 Journal of the Linnean Society, Mr. Baker says : — 



"The subjects of the differential characters, the rela- 

 tion diip to one another, and the climatic and geographical 

 individuality of the numerous types of tuber-bearing 

 Solanums are of great interest both from a botanical and 

 economic point of view. As there are many points which 

 are still to be unravelled, I propose in the present paper 

 to pass in review the material which we possess in Eng- 

 land bearing on the question. It was at the instigation 

 of Earl Cathcart that I undertook the inquiry ; and in 

 carrying it out I have gone through all the dried speci- 

 mens at Kew, the British Museum, and the Lindley Her- 

 barium, have carefully studied the wild types which we 

 grow in the herbaceous ground at Kew, and have visited 

 the extensive trial-grounds of Messrs. Sutton and Sons 

 at Reading, whose collection of cultivated types in a 

 1 ving stale is probably the most complete in existence." 



Bearing in mind that the potato, the most productive 

 of our food-plants, has become the most uncertain among 

 them in regard to its annual produce, it is not surprising 

 that it should have been the subject of v oluminous writing 

 and continual inquiry. But, in spite of all the pains 

 which have been expended on this stricken esculent, no 

 one but Mr. Baker seems to have recognised the outrage 

 of in-and-in breeding to which it has been subjected. It 

 seems doubtful wdiether the numerous breeders were 

 aware that the cultivated potato had been made the sub- 

 ject of continual in-and-in breeding, since it had never 

 been crossed out of its own family during the 250 years 

 of its highly artificial treatment in this country as a cul- 

 tivated plant. Yet this has been the case, as Mr. Baker 

 shows in his enumeration of the six tuber-bearing species 

 of the plant. As the habitat as well as the distinctions 

 of species of Solanum affect the subject, the following 

 brief details have been taken from Mr. Baker's paper : — 



"(1) .S'. tuberosum. — Andes of Chili, Peru, Bolivia, 

 Ecuador, and Columbia ; also in the mountains of Costa 

 Rica, Mexico, and the South-Western United States. 



" (2) k S'. Maglia. — Shore of Chili, down south as far as 

 the Chonos Archipelago ; also likely Peru. 



"(3) S. Commersoni. — Uruguay, Buenos Ayres, and 

 Argentine Territory, in rocky and and situations at a low 

 level. 



" (4) S.cardiophyllum. — Mountains of Central Mexico, 

 at an elevation of 8000 to 9000 feet. 



" (5) S. Jamesii.- -Mountains of South- Western United 

 States and Mexico. 



" (6) .S\ oxycarpum. — Mountains of Central Mexico." 



According to Bentham and Hooker, the great genus of 

 Solanum — the largest in the world- consists of decidedly 

 distinct species, and if we omit some of the so-called 

 species which are really only varieties of S. tuberosum, 

 these six species alone bear tubers. 



In attempting improvement by crossing the cultivated 

 potato, it is useless to continue the system of interbreed- 

 ing with its own varieties ; and, on the other hand, the 

 lesser forms of wild potatoes, such as S. Jamesii, a plant 

 of eight or ten inches in height, must be rejected. Mr. 

 Baker recommends two species as best for the breeders' 

 purpose, .V. Maglia and -S". Commersoni. Both these kinds 

 yield good crops of fair quality under cultivation, and they 

 possess the advantage of coming from a moist climate. 

 This is a point of great importance. When Mr. Darwin, 

 a young naturalist in 1835, was writing his account of 

 "The Voyage of the fr'cag/e," he mentioned having seen 

 the potato growing wild on the shores of the islands of the 

 Chonos Archipelago, in South America, and he thought 

 it surprising that the same plant should be found in the 

 damp forests of those islands and on the sterile mountains 

 of Central Chili, where a drop of rain does not fall for 

 more than six months. The explanation of this anomalous 

 circumstance is that the potato of the islands and low- 

 lands belongs to a different species from that of the 

 mountains, the latter being identical with the cultivated 

 potato of Europe and America, while the former is S. 

 Maglia, which is at any rate hardy, vigorous, and healthy, 

 and 111 all respects apparently well suited for crossing with 

 the cultivated sorts. This is the potato which Mr. Baker 

 recommends. Earl Cathcart had asked him for any sug- 

 resl ions that a botanist might be able to offer to breeders 

 founded upon scientific knowledge of the potato generally 

 and of the geographical distribution of the family. 



On this part of his inquiry, Mr. Baker observes 

 that potato-growers work upon the assumption that 

 the one purpose of the plant's existence is the production 

 of potatoes, which is in fact only an incident in its life. 

 .v. Maglia has been grown at Kew among the herbaceous 

 plants since 1S62, and in that dry sandy soil, without 

 manure, it produces few if any tubers, or only of small 

 size. On the other hand, two tubers were sent to Chis- 

 \\ ii k and grown there in the gardens of the Royal Horti- 



