Chapter 32

the effects of the removal of the anthers see Mr. Leitner in Silliman's 'North American Journ. of Science' volume 23 page 47; and Verlot 'Des Varietes' 1865 page 84.) Long-continued cultivation in rich soil seems to be the commonest exciting cause. A double narcissus and a double Anthemis nobilis, transplanted into very poor soil, has been observed to become single (18/99. Lindley's 'Theory of Horticulture' page 3?3.); and I have seen a completely double white primrose rendered permanently single by being divided and transplanted whilst in full flower. It has been observed by Professor E. Morren that doubleness of the flowers and variegation of the leaves are antagonistic states; but so many exceptions to the rule have lately been recorded (18/100. 'Gardener's Chronicle' 1865 page 626; 1866 pages 290, 730; and Verlot 'Des Varietes' page 75.), that, though general, it cannot be looked at as invariable. Variegation seems generally to result from a feeble or atrophied condition of the plant, and a large proportion of the seedlings raised from parents, if both are variegated, usually perish at an early age; hence we may perhaps infer that doubleness, which is the antagonistic state, commonly arises from a plethoric condition. On the other hand, extremely poor soil sometimes, though rarely, appears to cause doubleness: I formerly described (18/101. 'Gardener's Chronicle' 1843 page 628. In this article I suggested the theory above given on the doubleness of flowers. This view is adopted by Carriere 'Production et Fix. des Varietes' 1865 page 67.) some completely double, bud-like, flowers produced in large numbers by stunted wild plants of Gentiana amarella growing on a poor chalky bank. I have also noticed a distinct tendency to doubleness in the flowers of a Ranunculus, Horse-chestnut, and Bladder-nut (Ranunculus repens, Aesculus pavia, and Staphylea), growing under very unfavourable conditions. Professor Lehmann (18/102. Quoted by Gartner 'Bastarderzeugung' s. 567.) found several wild plants growing near a hot spring with double flowers. With respect to the cause of doubleness, which arises, as we see, under widely different circumstances, I shall presently attempt to show that the most probable view is that unnatural conditions first give a tendency to sterility, and that then, on the principle of compensation, as the reproductive organs do not perform their proper functions, they either become developed into petals, or additional petals are formed. This view has lately been supported by Mr. Laxton (18/103. 'Gardener's Chronicle' 1866 page 901.) who advances the case of some common peas, which, after long-continued heavy rain, flowered a second time, and produced double flowers.

SEEDLESS FRUIT.

Many of our most valuable fruits, although consisting in a homological sense of widely different organs, are either quite sterile, or produce extremely few seeds. This is notoriously the case with our best pears, grapes, and figs, with the pine-apple, banana, bread-fruit, pomegranate, azarole, date-palms, and some members of the orange-tribe. Poorer varieties of these same fruits either habitually or occasionally yield seed. (18/104. Lindley 'Theory of Horticulture' pages 175-179; Godron 'De l'Espece' tome 2 page 106; Pickering 'Races of Man;' Gallesio 'Teoria della Riproduzione' l816 pages 101-110. Meyen 'Reise um Erde' Th. 2 s. 214 states that at Manilla one variety of the banana is full of seeds: and Chamisso (Hooker's 'Bot. Misc.' volume 1 page 310) describes a variety of the bread-fruit in the Mariana Islands with small fruit, containing seeds which are frequently perfect. Burnes in his 'Travels in Bokhara' remarks on the pomegranate seeding in Mazenderan, as a remarkable peculiarity.) Most horticulturists look at the great size and anomalous development of the fruit as the cause, and sterility as the result; but the opposite view, as we shall presently see, is more probable.

STERILITY FROM THE EXCESSIVE DEVELOPMENT OF THE ORGANS OF GROWTH OR VEGETATION.

Plants which from any cause grow too luxuriantly, and produce leaves, stems, runners, suckers, tubers, bulbs, etc., in excess, sometimes do not flower, or if they flower do not yield seed. To make European vegetables under the hot climate of India yield seed, it is necessary to check their growth; and, when one-third grown, they are taken up, and their stems and tap-roots are cut or mutilated. (18/105. Ingledew in 'Transact. of Agricult. and Hort. Soc. of India' volume 2.) So it is with hybrids; for instance, Prof. Lecoq (18/106. 'De la Fecondation' 1862 page 308.) had three plants of Mirabilis, which, though they grew luxuriantly and flowered, were quite sterile; but after beating one with a stick until a few branches alone were left, these at once yielded good seed. The sugar-cane, which grows vigorously and produces a large supply of succulent stems, never, according to various observers, bears seed in the West Indies, Malaga, India, Cochin China, Mauritius, or the Malay Archipelago. (18/107. Hooker 'Bot. Misc.' volume 1 page 99; Gallesio 'Teoria della Riproduzione' page 110. Dr. J. de Cordemoy in 'Transact. of the R. Soc. of Mauritius' new series volume 6 1873 pages 60-67, gives a large number of cases of plants which never seed, including several species indigenous in Mauritius.) Plants which produce a large number of tubers are apt to be sterile, as occurs, to a certain extent, with the common potato; and Mr. Fortune informs me that the sweet potato (Convolvulus batatas) in China never, as far as he has seen, yields seed. Dr. Royle remarks (18/108. 'Transact. Linn. Soc.' volume 17 page 563.) that in India the Agave vivipara, when grown in rich soil, invariably produces bulbs, but no seeds; whilst a poor soil and dry climate lead to an opposite result. In China, according to Mr. Fortune, an extraordinary number of little bulbs are developed in the axils of the leaves of the yam, and this plant does not bear seed. Whether in these cases, as in those of double flowers and seedless fruit, sexual sterility from changed conditions of life is the primary cause which leads to the excessive development of the organs of vegetation, is doubtful; though some evidence might be advanced in favour of this view. It is perhaps a more probable view that plants which propagate themselves largely by one method, namely by buds, have not sufficient vital power or organised matter for the other method of sexual generation.

Several distinguished botanists and good practical judges believe that long- continued propagation by cuttings, runners, tubers, bulbs, etc., independently of any excessive development of these parts, is the cause of many plants failing to produce flowers, or producing only barren flowers,--it is as if they had lost the habit of sexual generation. (18/109. Godron 'De l'Espece' tome 2 page 106; Herbert on Crocus in 'Journal of Hort. Soc.' volume 1 1846 page 254: Dr. Wight, from what he has seen in India, believes in this view; 'Madras Journal of Lit. and Science' volume 4 1836 page 61.) That many plants when thus propagated are sterile there can be no doubt, but as to whether the long continuance of this form of propagation is the actual cause of their sterility, I will not venture, from the want of sufficient evidence, to express an opinion.

That plants may be propagated for long periods by buds, without the aid of sexual generation, we may safely infer from this being the case with many plants which must have long survived in a state of nature. As I have had occasion before to allude to this subject, I will here give such cases as I have collected. Many alpine plants ascend mountains beyond the height at which they can produce seed. (18/110. Wahlenberg specifies eight species in this state on the Lapland Alps: see Appendix to Linnaeus 'Tour in Lapland' translated by Sir J.E. Smith volume 2 pages 274-280.) Certain species of Poa and Festuca, when growing on mountain-pastures, propagate themselves, as I hear from Mr. Bentham, almost exclusively by bulblets. Kalm gives a more curious instance (18/111. 'Travels in North America' English translation