77 



If we observe for some time the development of different 

 kinds^of cuttings in a saturated atmosphere, it vail be seen 

 that the formation of the lenticel-e^corescences here described 

 illustrates the first of those formations which their tissues' 

 experience under the influence of moist air. In some species „ 

 the lenticels widen into rifts running lengthwise, which ex- 

 pose in places the bark tissue of the cuttings and which can 

 enlarge to several centimetres long and up to one centimetre 

 wide. At the same tim.e the bark sv/ells up greatly. ('Compare 

 fig, 18.) All these changes arise from the hypertrophic 

 growth of the bark cells. We v/ill later speak briefly of 

 "bark"" excrescences in the description of these processes of 

 growth. 



The plant, in which I could obtain experimentally the 

 most extensive bark-excrescences is Ri bes aure umo Since 

 this species is used as stock for stana.ard gooseberry and 

 currant bushes and since, further, in this species the de- 

 scribed excrescences may be observed in nature without 

 forced experimental interference, practica^i v.'Grker& h^ve given 

 repeated attention to this bark disease wh'.cl- 5 vcouer-^ design 

 nates as "dropsy"' ) or oedema). As figure 18 shov.s, boes- 

 shaped swellings are form.e4 on the diseased twlgz v/hich at 

 first are still surrounded by the cork- covering "but later ex- 

 pose their inner tissues in gaping rifts. Wounds of increas- 

 ing length and breadth are produced, until at last the swollen 

 bark tissue dies and collapses, figure 19 gives, a cross-sec- 

 tion through part of a boss-like bark excrescence still cov- 

 ered by cork. The parenchymatic cells of the bark have grown 

 out into long, sac-like cells of different form and size by 

 a perceptible growth in a radial direction, their length here 



(80) and there reaches ten or tv/elve times their width. At times, 

 scattered between the sacs radially arranged, may be found 

 elements which have been- stretched tangentially. The cells 

 of the bark medulkary rays also take part in the hypertrophic 

 gro\vth. It should therefore be observed that, not only the 

 cells of the outermost bark layers, but all the zones even 



up to the vrood itself, can participate -in the abnormal grov/th. 



All hypertropiiied elements have become completely or 

 nearly colorless, the chlorophyll has disappeared, the firm 

 connection betv;een the bark cells Ife lost^ they are separated 

 everyv/here from one another by large intercellular spaces, 

 the air-content of which gives the exposed bark tissue its 

 snov;^: lustre; in places the tissue disintegrates completely 

 into its individual elements. The cell walls are most deli- 

 cate, the contents consist of a thin layer of cytoplasm, and 

 a large, clear vacuole. The bark excrescences consist there- 

 fore of a tissue which corresponds in all essential points 

 with the aerenchyma-like products of the lenticels:- a pro- 



(81) nounced kataplastic hypertrophy is involved in their production. 

 There lie here and there betv/een the greatly enlarged paren- 

 chyma cells grop-ps of prosenchsrniatic elements which do not take 



; part in the enlargement, 



•1. Compare especially Handb. d. Pflanzenkrankh. , ^ Aufl., 

 1886, Bd, I, p. 233. 



