SECTIONAL TRANSACTIONS.— K. 395 



distinct forms, this may also be claimed for intraspecific hybrids which are generally 

 included in the term " variations.' Thus we come back to Weismann's view that 

 Amphimixis confers an essential advantage in the modification of species. 



Lotsy and Goddijn have in a recent work indicated forty-three South African 

 plant-hybrids which are all, according to their view, interspecific. Strictly speaking 

 they are all assumed hybrids, but this may pass. Forms assumed to be hybrids on 

 fairlj- good grounds seem to be just as common in South Africa a.s in other countries. 

 The writer is in a position to add a number to the list just referred to. 



Granted that hybrids may represent or lead to well-defined new forms, the claim 

 that hybridisation is an important factor in evolution seems to be well founded. 

 Lotsy thinks it is the only factor, but judgment has, for the present at all events, to 

 be held in suspense chiefly for the following reasons : — 



(1) In spite of the large number of hybrids known in well-explored European 

 countries, the species have for a long time remained unchanged. Very few new 

 species have been discovered since intensive work has been devoted to them, and 

 only in one case known to the writer has a hybrid origin been made likely. In 

 South Africa we can also show that a number of species have persisted for a couple 

 of hundred years, and in a few cases this has happened to species which hybridise 

 freely in nature. 



(2) Phytogeography and Palaeontology afford numerous instances proving that 

 species have persisted for thousands of years and even through geological ages, 

 which one would not expect if hybridisation had the modifying influence claimed 

 for it. 



(3) Hybridisation need not necessarily be the only cause of evolution. Apogamous 

 species vary just as much as sexually produced species. Innate causes, or causes 

 which appear to be innate owing to our inability to analyse them, may be at play in 

 creating new species. The known facts of bud-sports may indicate that new species 

 may arise in the course of vegetative reproduction. 



(4) In large groups of plants Orthogenesis appears to be responsible for evolution. 

 This also seems to be the case in certain genera in the evolution of species of which 

 the writer quotes some instances. 



(5) Natural selection reigns supreme, no matter what the origin of new forms is. 

 Co-adaptation (in Herbert Spencer's sense) cannot be expected to occur often (if at 

 aU) in interspecific hybrids, while this may happen more frequently in intraspecific 

 hybrids. The latter seem, therefore, to have an advantage over the former in the 

 struggle for existence. 



Dr. Ethel M. Doidge.- — Some Diseases of Citrus prevalent in South Africa. 



The diseases considered are those responsible for wastage in citrus fruit exported 

 from South Africa. 



The organisms responsible for the greater part of the wastage are the common 

 moulds {PenicilUum spp.) and Alternaria citri. Some account is given of recent 

 packing experiments with fruit which had been dipped in antiseptic solutions as 

 compared with fruit packed in specially treated wrappers. A summary is also given 

 of work recently done in connection with Alternaria citri. 



A brief account is given of organisms of minor or periodic occurrence in citrus 

 fruits, and also of some blemishes which lead to breakdown of the rind and subsequent 

 rotting by mould fungi. 



Mr. J. E. VAN DEE Plank. — A Physiological Study of Concentric Ring 

 Blotch of Citrus. 



There is evidence to show that concentric ring blotch is probably due to 

 physiological causes. 



It is confined almost exclusively to the summer flush of growth, and only young 

 shoots are affected. It is chiefly a disease of nursery plants and young trees, the 

 young growth of older trees being more resistant. 



It has been found that the diseased leaves and twigs are deficient in phosphorus 

 as compared with healthy material. This is possibly partly due to a phosphorus 

 deficiency in the soil, but is largely due to excessive growth. Several pieces of 

 evidence indicate this. 



(1) It is chiefly a disease of nursery plants, where the ' eflSciency index ' of growth 

 production is greatest. 



