﻿I 66 FIELD AND FOREST. 



sible, seemed to indicate as a leading idea that all galls result from 

 an effusion of sap in the affected plant cells. They may be classified 

 as follows : 



I. Galls of unlimited growth in which, though commenced by an 

 adult individual, many generations of insects live and propagate. 



II. Galls of limited growth only inhabited by larvae. 



III. Malformation of flowers or fruit. 



In the first class we have two orders, internal and external galls. 



Order I. External galls in which the foreign organism clings to the 

 outside of the organ on which it grows, air is freely admitted to it, 

 and its form sometimes admits of considerable motion. 



Family I. Galls of the Hemiptera. In this family I include leaf 

 distortions caused by swellings on one side, which embrace two note- 

 worthy kinds of heteromorphism, that is the formation of two kinds of 

 galls on the same plant by the same insect, as in 



(a.) Phylloxera vastatrix, Planchon, on Fills vinifera which forms 

 leaf galls or root galls. 



(b.) The four sorts ot Br achy s cells Schrad (a species of coccus] on 

 Eucalyptus hosmastoma in Australia. The gall produced by each fe- 

 male insect is covered with a lid, but those of the males are open cups 

 on the leaves, containing many individuals. 



The effect of the insect's work sometimes extends to a distance, as in 

 the greening (/. e. green-bark) of Psylla fedioz, Frst., on Valcrianclla 

 olitoria or the woody petioles of Ulmus campestris when the leaves are 

 affected by the blisters of Schizoneura lanuginosa, Htg. The forma- 

 tion of galls on cryptogamous plants by Hemiptera is entirely unknown. 

 Among Monocotyledons we have only the flowering stems of Juncus 

 affected by Livia juncorum, Str. 



Group I. Galls of centrifugal development. The direction of the 

 growth of main axis being changed by the inhabitant of the gall — 

 diffuse in the case of Chermes. 



Series I. Simple galls. Conversion of a single organ to the gall. 

 (A.) Various distortions of leaves, ( 1 6.) (D.) Hollow galls with active 

 (B.) Torsion of the stem, (3.) cell formation, (23.) 



(C.) Local swellings, (4.) (-E-) Hypertrophy of flower, (2.) 



Series II. Collective galls. — Leaves and stems enter at the same 

 time into the formation of the gall. 



(A.) Galls on buds, usually rosette-like with great shortening of the 



