pea refuse should be gathered together and burnt, for it is in this material that the resting-spores hibernate during the winter, and burst into new growth in the early spring. Resting-spores are not destroyed by passing through the stomach of an animal as food. P. vicia, Berk., is a distinctly early species as distinguished from P. infestans, Mont., the fungus of the potato disease, which is a distinctly late one. Being early, the late crops of peas are seldom affected by this parasite; they are, however, frequently destroyed by the fungus next described. We have seen P. viciæ, Berk., growing within the pods of garden peas and upon the contained seeds. This parasite has been detected on the bush-vetch, Vicia sepium, L.; the common vetch, V. Sativa, L.; slender vetch, V. tetrasperma, Moench.; Lathyrus macrorrhizus, Wimm.; and on many species of Pisum and Orobus. CHAPTER XXXIII. PEA MILDEW. Erysiphe Martii, Lk. THE description given of Erysiphe graminis, D.C., the blight or mildew of grass, applies generally to Erysiphe Martii, Lk., of peas, beans, Umbelliferæ, etc. The species is named in honour of Martius, the famous botanist, and is the same with E. Pisi, Grev. The general appearance of the fungus, as seen upon a X.100 FIG. 123. PEA MILDEW. Conceptacle of Erysiphe Martii, Lk., on leaf of pea. leaf of the pea under the microscope, is shown, enlarged 100 diameters, in Fig. 123. The dark body in the centre of the illustration is the conceptacle of the fungus, with its jointed tentacle-like appendages. The involved pattern in outline shows the epidermis of the leaf with its numerous stomata as at AAA. The conceptacle or perithecium is distinctly smaller in size than in E. graminis. D.C., as illustrated in Fig. 57, and the cells forming the bark of the conceptacle are larger. Each conceptacle contains four or eight sporangia or spore cases, and each sporangium carries four to eight sporidia, as shown at Fig. 124, enlarged 500 diameters; this illustration may be compared with Fig. 60. The conceptacles arise from a dense woven mass of very fine white mycelium, omitted in Fig. 123 in favour of the epidermis of the leaf with its stomata. This mycelium is woven all over the stomata, and so one of the chief vital functions of the leaf, the transpiration of vapour, is arrested. The fungus grows on both sides of the leaves. X500 FIG. 124. PEA MILDEW. Sporangium or ascus, with spores, of Erysiphe Martii, Lk. Enlarged 500 diameters. This destructive blight of peas invariably invades the late varieties, and is especially virulent in dry seasons. In small gardens the attack of the fungus may be prevented by keeping the peas well supplied with water. This treatment, however, cannot be adopted in the fields, and watering favours the growth of the Peronospora last described. Sometimes late peas are so badly attacked by this fungus that they appear as if thickly dusted with powdered chalk, and on the white surface thus formed the innumerable black conceptacles of the Erysiphe may be readily seen with the unaided eye. An attack of this fungus generally stops the growth of the invaded plants, and makes the production of pods impossible. The mycelium is provided with the minute suckers termed haustoria, and these haustoria pierce the epidermis of the attacked plant and cause decay. We have seen this fungus growing with its conceptacles inside the pods of peas. It unfortunately happens that this parasite is not confined to peas; it sometimes grows on beans and melilot, plants belonging to the same family with the pea. It is, however, able to support itself on plants of St. John's wort, Hypericum; some umbelliferous plants, and the meadow-sweet, Spiræa Ulmaria, L. It is thus capable of growing effectually upon plants belonging to four different natural orders. The only known preventive against attacks of this pest is the destruction by fire of all invaded material. The fungus is preserved in decaying refuse; in this material the little brown conceptacles remain intact during the winter, and in the following summer they burst, and each example discharges about fifty little living transparent spores, as shown in Fig. 124, ready to grow on the leaves of peas, weave a spider-weblike mycelium over the surface, and pierce the epidermis. CHAPTER XXXIV. LETTUCE MILDEW. Peronospora ganglioniformis, Berk. THE putrefactive fungus of lettuces was detected by the Rev. M. J. Berkeley, and described and illustrated by him in vol. i. of the Royal Horticultural Society, 1846. The specific named ganglioniformis refers to the resemblance of the fruiting-threads of the fungus to the natural enlargements termed ganglions in the course of a nerve. Tulasne thought this parasite was a mere variety of P. parasitica, Pers., as found on cabbages and illustrated in this work at Fig. 29; but a reference to our illustration of P. ganglioniformis, Berk., enlarged at Fig. 125 to 400 diameters, as compared with P. parasitica, Pers., enlarged in Fig. 29 to 200 diameters, will show how distinct the two are from each other. Professor de Bary disapproved of Mr. Berkeley's specific name ganglioniformis, and substituted gangliformis for it, considering the latter more correct, but no alteration was required. Had it been necessary, the word gangliiformis, as printed by Dr. Max. Cornu, would be most correct. In both P. ganglioniformis, Berk., and P. parasitica, Pers., the fruitingstems or conidiophores and branches are flattened, and as these flattened stems and branches twist a little in the process of growth, they have a spurious appearance of swelling in a ganglionic manner. Each ultimate branchlet of P. ganglioniformis, Berk., is beautifully dilated into a saucer-like expansion, with a single excessively-attenuated spicule growing from the centre of the saucer, and with from three to five similar minute spicules growing round |