42 REACTION OF HOST TO PARASITIC ATTACK. 



ductive mycelium of the following forms also grows only under 

 the cuticle : Rhytisma andromedae, the spermogonial mycelium 

 of Piiccinia anemones, Phragniidium, and other Uredineae. 



In many cases of hypertrophy the epidermal cells become 

 enlarged in a radial direction, and this, as in Taphrina aurea, 

 may be accompanied by considerable thickening of the walls. 

 In other cases, like that produced by Synchytrium, the epidermal 

 cells may become gelatinous. 



The cork becomes abnormally increased in many examples of 

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

 epiphyllus a phelloderm is formed, while on normal twigs phellem 

 alone is produced. Cork is found in juniper needles with 

 Gynfinosporangium juniperinuin, though never in the normal needles. 

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

 hawthorn, deformed by Roestelia 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 bundles of sclerenchyma, and permeates the 

 medullary rays. 



Collenchyma was found by Wakker to be absent in all cases 

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

 for example, in stems and petioles of cowberry attacked ' by 

 Exdbasidium, stems of buckthorn with Aecidium rhamni, of 

 Crataegus with Roestelia lacerata, of nettle with Aecidium urticae, 

 and of Sanguisorbia with Xenodochus carhonariios. On stalks of 

 Umbelliferae with pustules of Protomyces, I found, where the 

 collenchyma region was involved, that that tissue was not 

 developed (Fig. 46). 



In all cases of hypertrophy, parenchyma plays an important 

 part. Most 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 Plasmodiophora, consist almost exclusively of parenchyma. 

 Thickening of stems or branches is generally due to increase 

 of the rind-parenchyma, as in buckthorn under influence of 

 Aecidium rhamni, hawthorn with Gymnosporangium clavariae- 

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



