3o DISEASES OF CULTIVATED PLANTS 



trimmed and the hole filled with cement to keep out wet and 

 fungus spores, otherwise the tree will soon become decayed 

 and hollow. 



DROUGHT 



Stagheaded trees. This disease is indicated by the top- 

 most branches of the crown dying, the dead branches showing 

 conspicuously above the general mass of foliage. The injury 

 is due to lack of water, which implies a shortage of food- 

 material, and the upper branches are starved, the lower ones 

 monopolising all the water and food. This may be owing to 

 prolonged drought, excessive drainage, or to any surrounding 

 conditions affecting the underground water-level. It is more 

 frequently due to the removal of litter or thinning or removal 

 of the undergrowth. Hartig says that ' when oaks that have 

 grown up in a dense wood of beeches, and that have but 

 poorly developed crowns in consequence, are isolated by the 

 removal of the beeches, they clothe their stems abundantly 

 with epicornic branches. For some years these, as well as the 

 crowns, thrive perfectly satisfactorily. In the process of time, 

 however, and especially on the lighter classes of soil which 

 are subject to rapid drought or are liable to produce weeds, 

 a portion of the topmost branches of the crowns die, and the 

 oaks become stagheaded. If the ground is protected in time 

 by under-planting, the top branches either do not die or the 

 disease fails to make any progress, and the stagheaded con- 

 dition may entirely disappear owing to the dry branches 

 dropping off.' 



Hartig and Somerville, Text-Book of Diseases of Trees, 

 p. 270 (1894). 



INJURIES DUE TO FROST AND HAIL 



Spring frosts. When frost occurs late in the spring many 

 plants suffer from its effects that have survived without injury 

 more severe frost during the winter. This is owing to the 

 fact that during the winter, when the plant is not vegetatively 

 active, or before its leaves have expanded, the amount of 

 water in those parts most susceptible of injury is compare 

 tively small, whereas when active growth has commenced, the 

 leaves and youngest shoots contain a considerable quantity 



