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THE AMERICAN NATURALIST [Vol. XL1I 



That parasitic fungi are able to produce an alteration 

 of form in plants has long been known. One of the most 

 familiar abnormal growths from such a cause is what is 

 commonly termed a witch's broom, so often observed on 

 evergreens. It is due to the action of species of Exoascus 

 and oEcidum, which induce the formation of a large num- 

 ber of adventitious buds within a comparatively short 

 area of the stem or branch, which give rise to a corre- 

 sponding number of short, thickened twigs. In the silver 

 fir, Abies pectinata, witches brooms are produced by 

 Mcidium elatinum. 37 Frequently galls are produced by 

 fungi, affecting either roots, stems or leaves, but no cases 

 are on record where a fungus was shown to be the cause 

 of fasciation. This is different where gall-insects are 

 concerned. Here some cases have been traced directly to 

 gall-insects 38 as the cause. 



Galls, otherwise known as cecidia, and distinguished 

 according to their origin into zoo- and phytocecidia, are 

 among the most interesting of the abnormal forms which 

 from time to time make their appearance as excrescences 

 of widely varying shape, color and structure. Eecognized 

 by Pliny, some were even in those early days used in 

 medicine because of their astringent properties. To-day, 

 a number of them, especially some occurring on certain 

 species of Quercus, Pistacia, Ehus and Tamarix, are of 

 economic value 39 on account of their tannin content, and 

 a gall produced by Cynips tinctoria upon branches of the 

 dyer's oak, Quercus lusitanica (Q. infectoria), found in 

 the countries bordering the Mediterranean and in the 

 Orient, is official in the IT. S. Pharmacopoeia. Members of 

 widely different orders of insects may be the cause of the 



n Kerner, A., and Oliver, F. W. The Natural History of Plants, 2 : 527, 

 London, 1894-5. 



M Though gall insects only are discussed here it does not follow that 



between fasciation in species of Oenothera and the larvae of a small moth, 

 Mompha, is discussed in a very interesting, well illustrated paper by Knox, 

 The Plant World, 10 T : 145. 



89 Wiesner, J. Die Bohstoffe des Pflanzenreiches, 1 : 674, Leipzig, 1900. 



