GENERAL PRACTICE. DISEASES. 22? 



phosphoric acid, sulphuric acid, potash, lime, magnesia, and iron. It is, therefore, 

 desirable to understand and recognise the fact that all fruit trees are dependent upon 

 certain conditions for their health and the development of their valuable products ; also 

 it is equally important to have a right comprehension of the fact that diseases are not 

 the result of chance, but really growths that luxuriate under special conditions, and 

 these, as a rule, unfavourable to the trees. Indeed, a very large portion of the diseases 

 of fruit trees spring from direct injuries to the tissues, exhaustion, or deprivation of 

 their juices by fungi and insects. 



Fungi. There are very few fruit trees which are not known to support one or more 

 species of vegetable parasites. These, in the case of " blights," "mildews," " rusts," 

 and many affections of the stems, leaves, flowers, or fruit, are invisible, except as a 

 mass, to the unaided eye. Small as these microscopical fungi are, the botanist names 

 and classifies the species, describing their characteristics of form, life, and reproduction 

 in relation to the diseases and injuries of cultivated plants. Thus the cultivator is 

 made aware that "blights" and various diseases are the results of specific organic 

 growths, each producing characteristic effects. Each fungus produces its own seed 

 (spores), and from these alone is reproduction possible. It is of great importance to 

 fully comprehend that fungus spores always germinate outside the plant tissues, and 

 gain entrance by mechanically penetrating the epidermis and by wounds. They can- 

 not be taken up by the roots with water and carried with the latter to any part of the 

 plant. Entrance of the fungus is effected by piercing the surface, the germinal tube 

 accomplishing this by its power of absorbing the substance at the point of contact, or 

 by reaching and passing through the stomata. A thick epidermis is often a safeguard 

 against fungi, this alcno being sufficient to account for the immunity of certain varieties 

 from diseases which so nearly exterminate others. 



Knowledge on the conditions of germination is essential for comprehending the diseases 

 due to fungi. The spores consist each of a single cell, formed of an enclosed mass of 

 plastic substance (protoplasm), around which are two coats, the inner thin and flexible, 

 the outer usually thicker and much less elastic. Tn germination the outer is pierced 

 or cracked, and the inner coat protrudes as a long tube containing still the soft internal 

 substance. This tube is that which penetrates the plant, becoming perhaps a hundred 

 times as long as the spore before gaining access to the hypodermal tissues or those 

 beneath the cuticle or bark. Once the living tissues of the host, or tree affected with 

 parasitic fungus, are pierced, the mycelium or delicate transparent filaments (root fibres) 



G G 2 



