ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 499 



fungus penetrates the tuber at the point of its formation and the 

 infection spreads inwards, the diseased parts becoming brown ; this latter 

 disease is probably due to the association of F. oxysporum. 



Problems of Plant Pathology.*— F. L. Stevens writes on this 

 subject with regard to the fungi that form diseases. He quotes a 

 previous author as to the conception of three categories of plant 

 diseases : — 1. Those in which the parasites kill the living cells. 2. Those 

 in which it lives in association with them, feeding (5n their products. 

 3. Those in which the parasites invade the vessels and live in the sap. 

 Stevens has divided parasites into a more detailed series, which he has 

 shortly summarized as : — 1. The parasite living in the sap or in parts 

 devoid of protoplasm. 2. The parasite drawing its nutriment from 

 living cells. 3. The parasite living in cells that it has just killed. 

 Stevens points out the various aspects of plant pathology that are 

 imperfectly understood and that require to be studied. 



liicheus. 



(By A. LoERAiN Smith, F.L.S.) 



African Lichens. f — Carlo Zanfrogini has published a descriptive 

 list of lichens collected by Guido Paoli in Somali Land. Very full 

 notes of structural and other peculiarities are given. Eighteen species 

 or varieties have been thus fully described ; none of them are new nor 

 peculiarly African. Graphidine^ are well represeni-^d. 



Chgenotheca melanophaea var. flavocitrina.:[: — R. Paulson, who 

 discovered this new variety, found it covering large areas of the trunks 

 of various trees in Bricket Wood, Herts. It develops first under the 

 bark, and later emerges on the surface where the somewhat long-stalked 

 fruits are formed. The presence of salazinic acid — as is the species — 

 is proved by the purple reaction with potash. The host-cells in contact 

 with the lichen are mainly affected with the stain. 



Rate of Growth and Spreading (Ecesis) in Lichens. § — Bruce Fink 

 gives us a series of observations made over a period of eight years as to 

 the rate of growth of lichen thalli, and also as to period of time required 

 to re-establish a lichen on areas from which the plants had been removed. 

 Numerous results are given as to the rate of growth, most of them 

 about 1 cm. per year, or somewhat under. The greatest rate seems to 

 have been recorded for a plant of Feltigera caniiia growing on " a mossy 

 rock along a brook in a low moist wood, well-shaded." A plant, 



* Bot. Mag., Ixiii. (1917) pp. 297-306. 



t Nuova Notarisia, xxviii. (1917) pp. 145-75. 



X Journ. Bot,, ]v. (1917) pp. 197-8. 



§ Mycologia, ix. (1917) pp. 138-58. 



2 L 2 



