334 A TEXT-BOOK OF BOTANY. 



proximity their limbs will lash each other, causing a flattening 

 of the top which is very characteristic in the groves of trees on 

 the sea coast. In this way it is possible for the whip-like branches 

 of the birch to mutilate even the tops of the fir tree, changing 

 their spire-like summits to deliquescent crowns. Injuries causing 

 an alteration in the form of plants are also caused by ruminating 

 animals and leaf-devouring insects. 



Galls. — These are abnormal developments on the young twigs, 

 leaves, and flowers, being caused by the punctures and presence 

 of the deposited ova of quite a variety of insects. Galls vary in 

 si^e, form, and general appearance. They are only capable of 

 being produced either in meristematic cells or in tissues that are 

 capable of exercising this function. They are never formed on 

 Uiature stems, leaves, or flowers. The older parts may be eaten 

 and destroyed by insects, but they are not capable of being meta- 

 morphosed. In these growing tissues the mother insect lays her 

 eggs, which upon further growth, either through the secretion of 

 particular substances or otherwise, determine the direction of 

 growth of the cells and the final product which shall be formed. 



As has already been stated, galls show considerable variation, 

 and, as there are many hundreds of distinct galls, various attempts 

 have been made to classify them. Kiister has proposed an ana- 

 tomical classification as follows: i. Galls in which there may be 

 an enlargement of cells, but no cell multiplication ; 2. Soft galls, 

 composed of numerous cells, the resulting product being more or 

 less fleshy ; 3. Hard galls, in which there is an active cell division, 

 and sclerotic modification of the external layer so as to prevent 

 the drying up of the gall in summer and to guard against attack 

 by birds and other animals. Alodry (Beitrage zur Gallenbiologie), 

 on the basis of Kiister's classification, has given a very compre- 

 hensive review of the various structural (both external and in- 

 ternal) characters of the various groups of zoo-cecidia. 



In a classification of galls Thomas has suggested as a class 

 name for these structures the word cecidiex (meaning nut-gall). 

 The cecidien or galls are divided into two main groups, as fol- 

 lows : I. Phyto-cecidien or fungus galls, including the parasitic 

 fungi which cause a metamorphosis in the shoots of larger shrubs 

 and trees forming the structures commonlv known as " Witches' 



