3 + 



UREDINEAE 



Fi 



terminating in a chain of spores, the aecidiospores. It is these which are 

 able to infect the young wheat plant at the beginning of the season, hut after 

 that the same function is carried out by successive generations of uredospores. 



On the upper surface of the barberry leaf are a number of small flask-shaped 

 openings, appearing externally as the brown dots mentioned before, called 

 pyenidia. These, as shown in the figure, produce vast numbers of 

 exceedingly small cells which have been variously regarded as spores and as 

 male gametes. In any event these cells are functionless now, so far as we 

 know. It is probable that in South Africa uredospore infection is quite 

 sufficient to account for the disease, without an alternative host-plant being 

 necessary. Even in Europe, with a long cold winter, uredospore sori sometimes 



survive on winter wheats and wild grasses ; and 

 the conditions permitting such survival are much 

 more favourable here. 



As a South African example of the accidium- 



stage, Aecidium resinicolum [Plate 3, big. BJ has 



been chosen. This causes large galls on the stem 



and leaves of Rafnia angulata ; each gall is covered 



with numerous aecidium cups, which differ from 



those of Puccinia in having a long peridium pro- 



30. Pycnidium bom jecting from the surface of the gall. In early 



leaf of Rafnia angulata. stages of the development of the galls the aecidia 



200/1. w. r. s. (Fig- 2 9) are found on both surfaces of the leaf, 



and interspersed with them are pyenidia (Fig. 30), hardly distinguishable from 



those already described^and figured. 



LICHENES*. 



It is probable that most people recognise a lichen when they see one, 

 but few are aware of the peculiarities which enable a lichen to live where 

 no other plant can. The reason for this lies in the fact that every 

 lichen consists not of a single organism but of two, quite different in function 

 and structure, and mutually dependent upon each other for existence in 

 those dry and barren spots where lichens are often found. 



One of these organisms is an alga belonging to either the Schizophyceae 

 or the Chlorophyceae, and the other is a fungus, mostly of the Discomycetes. 

 Usually the algal constituent is also known in the tree state in more congenial 

 habitats ; but the fungal constituent is only rarely able to exist except in the 

 lichen form. Hence the name of the lichen is used either tor the whole plant 

 or for the fungal constituent, but not tor the alga, which has its own name. 

 A further reason for this is that it is only the fungus which forms the typical 

 reproductive cells of its class (ascospoves), while the alga remains in the 

 vegetative condition. 



The lichen-forming algae are often such as possess more or less mucilaginous 



* Numerous lichens occurring in South Africa are described or enumerated by 

 Massalongo, Stirton and S i izknueroer. See literature, page 246. 



