LICHENES 507 



solution of caustic soda, when the trees are in a resting 

 condition. 



In the tropics more especially, numerous species of lichens 

 form more or less extended pale-green or whitish patches on 

 the surface of evergreen, coriaceous leaves. Such species 

 are undoubtedly more or less parasitic in habit, but as a 

 rule do but little injury, unless present in sufficient quantity 

 to cover the greater portion of the leaf, which is thereby 

 prevented from performing its various functions. It has 

 already been stated that a lichen is composed of one or more 

 algae and a fungus, which work in unison. It would appear 

 that in the case of Cephaleuros mycoidea (Karsten), the red 

 rust of the tea plant, which is the most injurious lichen 

 known, the algal element lives independently for a consider- 

 able time, and during this period is the cause of the trouble. 

 When it combines with a fungus to form a true lichen it is 

 no longer directly injurious. 



Bed rust of the tea plant. The most destructive and at 

 the same time the most widely distributed of pests attacking 

 the tea plant in India. It is now known as Cephaleurus 

 mycoidea^ Karsten ( = Mycoidea parasitica^ Cunningham, 

 Cephaleurus virescens, Kunze). Although the ultimate 

 condition of this parasite is a true lichen bearing ascigerous 

 fruit, its algal constituent usually remains for a considerable 

 length of time perfectly free from the fungus component. In 

 this stage it appears under the form of livid red, or orange- 

 red spots of variable size on th.e leaves and branches. When 

 a red tuft is examined under a magnify ing-glass it is seen to 

 consist of numerous erect threads, each ending in a little 

 knob. In this form it is a decided parasite, and is capable 

 of killing the branches. On the leaves the direct amount of 

 injury is unimportant, but the spores produced enables 

 the disease to spread to neighbouring bushes. On the leaf 

 the patches are superficial and can be entirely removed with 

 a sharp knife. The red algal element sometimes produces its 

 own type of fruit, or it may be joined by a fungus and form 

 grey or whitish, more or less polished, patches of lichen 

 containing ascigerous fruit. On the stem the parasite behaves 

 in a totally different manner, and proves very destructive. 

 When once established the parasite penetrates the bark, 

 which, in consequence, peels off in thin flakes ; this process 

 continues until the living tissues are reached, when the sap 

 of the plant is absorbed by the parasite. 



