Most authors believe that all the races are descended from the wild cabbage found on the western shores of Europe; but Alph. De Candolle (9/74. 'Geograph. Bot.' page 840.) forcibly argues, on historical and other grounds, that it is more probable that two or three closely allied forms, generally ranked as distinct species, still living in the Mediterranean region, are the parents, now all commingled together, of the various cultivated kinds. In the same manner as we have often seen with domesticated animals, the supposed multiple origin of the cabbage throws no light on the characteristic differences between the cultivated forms. If our cabbages are the descendants of three or four distinct species, every trace of any sterility which may originally have existed between them is now lost, for none of the varieties can be kept distinct without scrupulous care to prevent intercrossing.
The other cultivated forms of the genus Brassica are descended, according to the view adopted by Godron and Metzger (9/75. Godron 'De l'Espece' tome 2 page 54; Metzger 'Kohlarten' s. 10.), from two species, B. napus and rapa; but according to other botanists from three species; whilst others again strongly suspect that all these forms, both wild and cultivated, ought to be ranked as a single species. Brassica napus has given rise to two large groups, namely, Swedish turnips (believed to be of hybrid origin) (9/76. 'Gardener's Chronicle and Agricult. Gazette' 1856 page 729. See more especially ibid 1868 page 275: the writer asserts that he planted a variety of cabbage (B. oleracea) close to turnips (B. rapa) and raised from the crossed seedlings true Swedish turnips. These latter plants ought, therefore, to be classed with cabbages or turnips, and not under B. napus.) and Colzas, the seeds of which yield oil. Brassica rapa (of Koch) has also given rise to two races, namely, common turnips and the oil-giving rape. The evidence is unusually clear that these latter plants, though so different in external appearance, belong to the same species; for the turnip has been observed by Koch and Godron to lose its thick roots in uncultivated soil; and when rape and turnips are sown together they cross to such a degree that scarcely a single plant comes true. (9/77. 'Gardener's Chronicle and Agricult. Gazette' 1855 page 730.) Metzger by culture converted the biennial or winter rape into the annual or summer rape,—varieties which have been thought by some authors to be specifically distinct. (9/78. Metzger, 'Kohlarten' s. 51.)
In the production of large, fleshy, turnip-like stems, we have a case of analogous variation in three forms which are generally considered as distinct species. But scarcely any modification seems so easily acquired as a succulent enlargement of the stem or root—that is, a store of nutriment laid up for the plant's own future use. We see this in our radishes, beet, and in the less generally known "turnip-rooted" celery, and in the finocchio, or Italian variety of the common fennel. Mr. Buckman has lately proved by his interesting experiments bow quickly the roots of the wild parsnip can be enlarged, as Vilmorin formerly proved in the case of the carrot. (9/79. These experiments by Vilmorin have been quoted by many writers. An eminent botanist, Prof. Decaisne, has lately expressed doubts on the subject from his own negative results, but these cannot be valued equally with positive results. On the other hand, M. Carriere has lately stated ('Gardener's Chronicle' 1865 page 1154), that he took seed from a wild carrot, growing far from any cultivated land, and even in the first generation the roots of his seedlings differed in being spindle-shaped, longer, softer, and less fibrous than those of the wild plant. From these seedlings he raised several distinct varieties.)
This latter plant, in its cultivated state, differs in scarcely any character from the wild English carrot, except in general luxuriance and in the size and quality of its roots; but ten varieties, differing in the colour, shape, and quality of the root, are cultivated in England and come true by seed. (9/80. Loudon's 'Encyclop. of Gardening' page 835.) Hence with the carrot, as in so many other cases, for instance with the numerous varieties and sub-varieties of the radish, that part of the plant which is valued by man, falsely appears alone to have varied. The truth is that variations in this part alone have been selected; and the seedlings inheriting a tendency to vary in the same way, analogous modifications have been again and again selected, until at last a great amount of change has been effected.
With respect to the radish, M. Carriere, by sowing the seed of the wild Raphanus raphanistrum in rich soil, and by continued selection during several generations, raised many varieties, closely like the cultivated radish (R. sativus) in their roots, as well as the wonderful Chinese variety, R. caudatus: (see 'Journal d'Agriculture pratique' tome 1 1869 page 159; also a separate essay 'Origine des Plantes Domestiques' 1869.) Raphanus raphanistrum and sativus have often been ranked as distinct species, and owing to differences in their fruit even as distinct genera; but Professor Hoffman ('Bot. Zeitung' 1872 page 482) has now shown that these differences, remarkable as they are, graduate away, the fruit of R. caudatus being intermediate. By cultivating R. raphanistrum during several generations (ibid 1873 page 9), Professor Hoffman also obtained plants bearing fruits like those of R. sativus.
PEA (Pisum sativum).
Most botanists look at the garden-pea as specifically distinct from the field-pea (P. arvense). The latter exists in a wild state in Southern Europe; but the aboriginal parent of the garden-pea has been found by one collector alone, as he states, in the Crimea. (9/81. Alph. De Candolle 'Geograph. Bot.' 960. Mr. Bentham 'Hort. Journal' volume 9 1855 page 141 believes that garden and field peas belong to the same species, and in this respect he differs from Dr. Targioni.) Andrew Knight crossed, as I am informed by the Rev. A. Fitch, the field-pea with a well-known garden variety, the Prussian pea, and the cross seems to have been perfectly fertile. Dr. Alefield has recently studied (9/82. 'Botanische Zeitung' 1860 s. 204.) the genus with care, and, after having cultivated about fifty varieties, concludes that certainly they all belong to the same species. It is an interesting fact already alluded to, that, according to O. Heer (9/83. 'Die Pflanzen der Pfahlbauten' 1866 s. 23.) the peas found in the lake-habitations of Switzerland of the Stone and Bronze ages, belong to an extinct variety, with exceedingly small seeds, allied to P. arvense or the field-pea. The varieties of the common garden-pea are numerous, and differ considerably from one another. For comparison I planted at the same time forty-one, English and French varieties. They differed greatly in height,— namely from between 6 and 12 inches to 8 feet (9/84. A variety called the Rounciva attains this height, as is stated by Mr. Gordon in 'Transact. Hort. Soc.' 2nd series volume 1 1835 page 374 from which paper I have taken some facts.),—in manner of growth, and in period of maturity. Some differ in general aspect even while only two or three inches in height. The stems of the Prussian pea are much branched. The tall kinds have larger leaves than the dwarf kinds, but not in strict proportion to their height:—HAIR'S DWARF MONMOUTH has very large leaves, and the POIS NAIN HATIF, and the moderately tall BLUE PRUSSIAN, have leaves about two-thirds of the size of the tallest kind. In the DANECROFT the leaflets are rather small and a little pointed; in the QUEEN OF DWARFS rather rounded; and in the QUEEN OF ENGLAND broad and large. In these three peas the slight differences in the shape of the leaves are accompanied by slight differences in colour, in the POIS GEANT SANS PARCHEMIN, which bears purple flowers, the leaflets in the young plant are edged with red; and in all the peas with purple flowers the stipules are marked with red.
In the different varieties, one, two, or several flowers in a small cluster, are borne on the same peduncle; and this is a difference which is considered of specific value in some of the Leguminosae. In all the varieties the flowers closely resemble each other except in colour and size. They are generally white, sometimes purple, but the colour is inconstant even in the same variety. In WARNER'S EMPEROR, which is a tall kind, the flowers are nearly double the size of the POIS NAIN HATIF; but HAIR'S DWARF MONMOUTH, which has large leaves, likewise has large flowers. The calyx in the VICTORIA MARROW is large, and in BISHOP'S LONG POD the sepals are rather narrow. In no other kind is there any difference in the flower.
The pods and seeds, which with natural species afford such constant characters, differ greatly in the cultivated varieties of the pea; and these are the valuable, and consequently the selected parts. SUGAR PEAS, or POIS SANS PARCHEMIN, are remarkable from their thin pods, which, whilst young, are cooked and eaten whole; and in this group, which, according to Mr. Gordon includes eleven sub-varieties, it is the pod which differs most; thus LEWIS'S NEGRO-PODDED PEA has a straight, broad, smooth, and dark- purple pod, with the husk not so thin as in the other kinds; the pod of another variety is extremely bowed; that of the POIS GEANT is much pointed at the extremity; and in the variety "A GRANDS COSSES" the peas are seen through the husk in so conspicuous a manner that the pod, especially when dry, can hardly at first be recognised as that of a pea.
In the ordinary varieties the pods also differ much in size;—in colour, that of WOODFORD'S GREEN MARROW being bright-green when dry, instead of pale brown, and that of the purple-podded pea being expressed by its name;- -in smoothness, that of DANECROFT being remarkably glossy, whereas that of the NE PLUS ULTRA is rugged; in being either nearly cylindrical, or broad and flat;—in being pointed at the end, as in THURSTON'S RELIANCE, or much truncated, as in the AMERICAN DWARF. In the AUVERGNE PEA the whole end of the pod is bowed upwards. In the QUEEN OF THE DWARFS and in SCIMITAR PEAS the pod is almost elliptic in shape. I here give drawings of the four most distinct pods produced by the plants cultivated by me.