Promycelium bears four sterigmata and four gonidia (or sporidia), which in favorable conditions pass back to the barberry, germinate, the tube enters between cells into the intercellular spaces of the host to produce the cluster-cup again, and thus the life cycle is completed.
410. Other examples of the rusts.—Some of the rusts do great injury to fruit trees and also to forest trees. The “cedar apples” are abnormal growths on the leaves and twigs of the cedar stimulated by the presence of the mycelium of a rust known as Gymnosporangium macropus. The teleutospores are two-celled and are formed in the tissue of the “cedar apple” or gall. The teleutosori are situated at quite regular intervals over the surface of the gall at small circular depressions, and can be easily seen in late autumn and during the winter. A quantity of gelatine is developed along with the teleutospores. In early spring with the warm spring rains the gelatinous substance accompanying the teleutospores swells greatly, and causes the teleutospores to ooze out in long, dull, orange-colored strings, which taper gradually to a slender point and bristle all over the “cedar apple.” Here the teleutospores germinate and produce the sporidia. The sporidia are carried to apple trees where they infect leaves and even the fruit, producing here the cluster-cups. There are no uredospores.
G. globosum is another species forming cedar apples, but the gelatinous strings of teleutospores are short and clavate, and the cluster-cups are formed on hawthorns. G. nidusavis forms “witches brooms” or “birds nests” in the branches of the cedar. The mycelium in the branches stimulates them to profuse branching so that numerous small branches are developed close together. The teleutosori form small pustules scattered over the branches. G. clavipes affects the branches of cedar only slightly deforming them or not at all, and the cluster-cups are formed on fruits, twigs, and leaves of the hawthorns or quinces, the cluster-cups being long, tubular, and orange in color.
[CHAPTER XXI.]
THE HIGHER FUNGI.
411. The series of the higher fungi.—Of these there are two large series. One of these is represented by the sac fungi, and the other by the mushrooms, a good example of which is the common mushroom (Agaricus campestris).
Sac Fungi (Ascomycetes).
412. The sac fungi may be represented by the “powdery mildews”; examples, uncinula, microsphæra, podosphæra, etc. [Fig. 225] is from a photograph of two willow leaves affected by one of these mildews. The leaves are first partly covered with a whitish growth of mycelium, and numerous chains of colorless gonidia are borne on short erect threads. The masses of gonidia give the leaf a powdery appearance. The mycelium lives on the outer surface of the leaf, but sends short haustoria into the epidermal cells.
413. Fruit bodies of the willow mildew.—On this same mycelium there appear later numerous black specks scattered over the affected places of the leaf. These are the fruit bodies (perithecia). If we scrape some of these from the leaf, and mount them in water for microscopic examination, we shall be able to see their structure. Examining these first with a low power of the microscope, each one is seen to be a rounded body, from which radiate numerous filaments, the appendages. Each one of these appendages is coiled at the end into the form of a little hook. Because of these hooked appendages this genus is called uncinula. This rounded body is the perithecium.