418 GROWTH OF THE SEED. office of depositing alburnous or woody matter; and as soon as this is formed, the sap, which had hitherto descended only through the cortical vessels, begins to ascend through the alburnum. The plumule, in consequence, elongates, its leaves enlarge and unfold, and a set of vessels which did not exist in the root are now brought into action. These, which I have called the central vessels, surround the medulla, and between it and the bark form a circle, upon which the alburnum is deposited by the bark in the form of wedges, or like the stones of an arch. Through these vessels, which diverge into the leaf-stalks, the sap ascends, and is dispensed through the vessels and parenchymatous substance of the leaf; and in this organ the fluid, recently absorbed from the soil, becomes converted into the sap, or blood of the plant: and as this fluid, during germination, descended from the cotyledons and seed-leaves of the plant, it now descends from its proper leaves, and adds in its course to the bulk of the stem and the growth of the roots. Alburnum is also deposited in the stem of the plant, below the proper leaves, as it was previously below the seed-leaves; and from this spring other ascending vessels, which give existence to, and feed, other leaves and buds." CHAPTER XIII. THE DISEASES OF FRUIT-BEARING PLANTS. I. GENERAL OBSERVATIONS — BERKELEY'S THEORY OF CLASSIFICATION -WEIGMANN CAUSES OF DISEASE -INSECTS. § II. DISEASES OF THE APPLE OF THE LEAF-FALL OF THE LEAF FUNGI CLADOS THE RASPBERRY -THE OF THE LEAF INSECTS 420 GENERAL OBSERVATIONS. BEETLE, THE PYRALIS, THE PROCRIS AMERICANA, THE HALTICA CHALYBEA, THE LEAF HOPPER, THE BOMBYX QUATA, THE PHILAMPELUS, THE CHOEROCAMEPA, THE RHYNCHITIS, FUNGI, COMMON MILDEW, ERYSIPHE, BOTRYTIS; OF THE FRUIT AND FLOWER-THE ROSE-BUG, SHANKING. § 1. — GENERAL OBSERVATIONS. THE diseases of plants have never received the attention which they deserve. Of latter years investigation has been directed to insects injurious to vegetation; but this has been done more in relation to entomology than to pathology. We had described a great number of diseases affecting fruit trees, and had classified them in such a manner as we had hoped might contribute to the progress of vegetable pathology, when we discovered that its introduction to this volume would increase its size beyond proper proportion. Therefore we are constrained to retain many of the results of our investigation on this topic, which we may hereafter issue in a separate form. In this chapter we insert only some leading facts on the more common diseases. Even these we must treat with special regard to practice, omitting, for the most part, those novelties and freaks of nature which should be noticed in a full pathological treatise. We shall not confine ourselves to a strictly philosophical method; because, in the cultivation of some plants, the very state which we desire to induce is one of disease, provided this be considered as a departure from the normal type. For instance, the CLASSIFICATION OF DISEASES. 421 enlargement of the flesh of a fruit, the entire or partial absence of seed, are the results of disease, strictly considered; and yet these conditions are very desirable in fruit culture. Therefore, practically, we will regard as disease whatever is unfavorable to the growth of the plant for the purpose intended. This view is taken by Mr. Berkeley, of England, who defines disease as "a deviation from the normal condition of species." He treats the subject practically only as it is unfavorable to these deviations from nature produced by cultivation. Species have no natural tendency to disease, because this might destroy them; yet individuals or varieties have such a tendency, which they manifest in whatever condition they may be placed, and which they transmit to their offspring. Berkeley thus classifies diseases: Internal, or Constitutional; External, or Accidental; Doubtful, or Conjectural. Weigmann arranged them thus: Those which affect the organs of Nutrition, of Respiration, and of Reproduction. But we shall adopt a classification better suited to the object of this chapter, and shall speak of the discases of different species of fruit according to the part of the plant which they affect. This arrangement will enable the fruitgrower to avail himself readily of the results of our investigation. While far the larger part of the diseases of plants may be rather mechanical and accidental than con 422 CONSTITUTIONAL DISEASE. stitutional, yet there is a large class of the latter which have not been sufficiently studied to be described in detail. They are undoubtedly those which result from causes analogous to inflammation in the animal system, taking that term in its largest acceptation; yet their proportion is small. In some cases, during the inflamed condition of the organ there is an increased degree of heat, like that so often accompanying the same state in animals; while in other cases no difference in the temperature is perceptible.1 M. Huber found when the heat of the atmosphere stood at twenty-one degrees centigrade, that the instrument, surrounded with spadices of the arum cordipolium, during the process of fecundation, rose as high as forty-two degrees. While plants have not as definite a form as animals, and it is therefore more difficult to detect malformation, yet they exhibit a peculiar sensitiveness to external influences during their growing season. Mons. J. De Jonghe, of Brussels, states the causes which produce disease in fruit trees as follows: 2 1st. Constitutional weakness of the variety. Some sorts are weak because the conditions in which they are placed are unfavorable; perhaps they originated in a warmer region, and the severity or vicissitudes of the climate induces disease. Other varieties are 1 Gross's Elements of Pathological Anatomy. 2 In a paper published in "Gardeners' Chronicle," vol. for 1857. |