42 KKAt rioN OF HOST TO I'AUASITK' ATTACK. 



duflive luyceliuiii of the t'ollowin-- forms also grows only under 

 the cuticle: lihytisma andruma/ac, the spermogonial niyeeliuni 

 of Pinrinia annnonc.% Phragmidhm, and other Uredineae. 



In many cases of hypertrophy the epidermal cells become 

 enlarged in a radial direction, and this, as in Tcq)hrina cmrea, 

 may be accompanied by considerable thickening of the walls. 

 In other cases, like that i)roduced by Si/ncJn/frium, the epidermal 

 cells may become gelatinous. 



The cork becomes abnormally increased in many examples of 

 hypertrophy. Thus in witches' broom of alder due to Exoascus 

 rpiphjilhis a phelloderm is formed, while on normal twigs phelleni 

 alone is produced. Cork is found in juniper needles with 

 Gijmnoqxrmngiinnjunipcrimim, though never in the normal needles. 

 On the other hand, cork-formation is suppressed in twigs of 

 hawthorn, deformed by Rocstelia lacerata. The so-called "wound- 

 cork" is constantly associated with attacks of parasitic fungi; it 

 separates diseased portions of rind and bast from sound, forms 

 sheaths round Ijundles of sclerenchyma, and permeates the 

 medullary rays. 



Collenchyma was found by Wakker to be absent in all cases 

 of hypertKjphy of parts of plants where it is normally present ; 

 for example, in stems and petioles of cowberry attacked by 

 Ki:uh((si(hinu, stems of buckthorn with Aecidinm rhamni, of 

 Cmtorfjus with RoeMclia lacerata, of nettle with Aecidinm nrticae, 

 and of Sa)if/uisorhia with Xenodochvs carhonarius. On stalks of 

 Umbelliferae with pustules of Protomyces, I found, where the 

 collenchyma region was involved, that that tissue was not 

 ileveloped (Fig. 46). 



In all cases of hypertropliy, parenchyma plays an important 

 part. ]\Iost abnormal outgrowths result from multiplication and 

 enlargement of the cells of the parenchyma, the formation of 

 mechanical tissues being more or less suppressed. Thus the 

 gigantic examples of hypertrophy exhibited by turnips infested 

 by PlasvnxJvtphora, consist almost exclusively of parenchyma. 

 Thickening of stems or branches is generally due to increase 

 of the rind-parenchyma, as in l»uckthorn under influence of 

 Arridiinn rJiainni, hawthorn with (ri/vDuisjMnuiirjium clavariae- 

 foTvir, in most witches' brooms, and in many other cases. In 



