433 



The warts are developed most abundantly at the place where the grapes 

 join the stem: large branches of the clusters usually remain smooth and, 

 as a rule, only some grapes show the disease. This is unimportant in con- 

 tinued dry weather, but with humidity makes for a development of parasites. 

 If then a sharp dry period follows, some of the very warted stems shrivel 

 and the grapes as well. 



Fig. 74 shows a cross-section through a warty grape stem which exhibits 

 the usual axillary structure and has some strikingly broad medullary rays 

 (ms) which divide the wood ring (h). In the bark we notice a regular 

 distribution of the hard bast groups (h) and in front of them the sieve 

 elements (s) with often thickly swollen walls. At o is indicated one of the 

 abundant crystals of calcium ox- 





^ 



alate. These occur at times as 

 small glands, at times as raphides. 

 The different stages of the forma- 

 tion of these corky warts are 

 shown at W. The wart-like ex- 

 crescences, which resemble len- 

 ticels, are produced by the radial 

 enlargement of some of the par- 

 enchyma cells lying immediately 

 beneath the epidermis or some- 

 what deeper ; and the consequent 

 outpushing of the outer skin. By 

 an increase of this process, which 

 does not preclude the dividing of 

 the elongated cells, an accumu- 

 lation of tissue is produced with a 

 corky covering which finally be- 

 comes brown and splits. By the 

 increase of the bark parenchyma 

 and the dying of the outermost 

 brown corked elements the large 

 warts are produced, the peripheral cell layers of which are pushed out from 

 each other in a saucer-shaped form. A distinct cork cambium is formed 

 connected with the dying bark of the outermost layers. This constantly 

 extends deeper into the bark of the stem. If the weather continues to be 

 cloudy, warm and damp, or if the grapes are too much hidden under the 

 foliage, the conditions are ideal for the development of fungi among which 

 may be noticed first of all Bofrytis c'merea. 



The phenomenon is especially frequent in greenhouses, and here the 

 close, moist atmosphere must be improved by ventilation and heat must be 

 provided at the same time. If the warty grape stems are found out of doors, 

 some of the foliage above the bunches of grapes must be removed and, after 

 each rain, the water retained by the foliage carefully shaken off. 



.-^ 



Fu 



4. ('ro^.-^-siiiiuii il[juug-h the warty 

 fruit stem of a grape. (Orig.) 



