Page:TheTreesOfGreatBritainAndIreland vol04B.djvu/377

The witches’ brooms, which are so common on birch trees, are generally sup- posed to be due to a fungus, Exoascus betulinus, the threads of which penetrate the young growing twigs, causing them to branch repeatedly and thus form large, irregular, nest-like clusters. Miss Ormerod,! however, states that these abnormal growths are caused by the development of unhealthy buds, which have been attacked by a gall-mite.? It is possible that in some cases it is the fungus, and in other cases the gall-mite, which is the cause of these witches’ brooms.

A large number of birches were killed in 1900 in Epping Forest by a fungus, identified, by Paulson,’ with MJe/anconts stilbostoma, Tulasne, which attacks the young growing branches.

This species is very variable in the wild state, both as regards the stature of the‘ tree and the shape, size, and pubescence of the foliage. A large number of varieties have been distinguished by Continental botanists, of which the nomen- clature is very confused; and as most of these are separated by inconstant characters and are of no value from the cultivator’s point of view, it will be sufficient here to refer the reader to the works of Willkomm, Winkler, and Schneider, where the different forms are fully dealt with. The following varieties are, however, worthy of note :—

1. In Alpine and northern localities this species is often met with as a small shrub with twisted branchlets, but with leaves very variable in character. This group of forms may be distinguished as var. tortuosa, Koehne, Deut. Dend, 109 (1893).

2. Var. Murithii, Gremli, Excursionsfl. f. d. Schweiz, 365 (1893).

B. Murithii, Gaudichaud, Fl. Helv. vi. 178 (1830); Christ, Ber. Schweiz. Bot. Ges. v. 16 (1895).

An Alpine shrub, occurring in the Bagnes valley, near Mauvoisin (Valais), in Fribourg, and in the Joux valley (Vaud) in Switzerland. This has broadly ovate or ovate-triangular leaves, with large simple serrations, and prominent reticulate venation beneath.

3. Var. denudata, Grenier et Godron, F2. France, iii. 147 (1855). This name may be given to a series of forms, characterised by rhombic leaves, cuneate at the base, and glabrous beneath or with only slight axil-tufts. This is often cultivated as var. pontica,* var. carpatica,’ var. odorata, etc., and is usually a tree of considerable size.

4. Var. urticifolia, Spach, in Ann. Sc. Nat., sér. 2, xv. 187 (1841); Schneider, Laubholzkunde, i. 117 (1904).

Betula urticifolia, Regel, in Mém. Soc. Nat. Mosc. xiii. 115 (1860); Willkomm, Forstliche Flora, 313 (1887); Winkler, Betulaceæ, 80 (1904).

1 Injurious Insects, 212 (1890).

2 Gillanders, Forest Entomology, 25 (1908), identifies the gall-mite with Eriophes rudis, Canestrini; and gives a figure of swollen buds on the branch of a birch tree. These had been found in close proximity to a witches’ broom.

3 Essex Naturalist, xi. 1, p. 273 (1901). Cf. also Nature, lxii. 599 (1900). Mr. Massee thinks that root-rot or unsuitable soil conditions, rather than the fungus, were the cause of death of these trees.

4 There is a good-sized healthy tree in the Botanic Garden at Glasnevin, under the name of B. alba, var. pontica, which was mentioned by Loudon as being, in 1838, thirty-five years old and 35 feet high.

5 The true var. carpatica (B. carpatica, Waldstein and Kitaibel, in Willdenow, Sp. Pl. iv. 464 (1805)) is a low tree allied to var. tortuosa, the distinctive characters of which are given in Schneider, op. cit. 119.