89 



produced v/hich resembled the cells of normal bark tissue. 

 Large, ftvell- preserved clalorophyll grains, lying in tlie byper- 

 trophied cells, were conspicuous," I could not observe cell- 

 division. In all others which I investigated, the chlorophyl 

 content went to pieces, just as in the cells of the hyper- 

 hydric tissues. AlT/ays in fagus only single cells hypertro- 

 phied, gowmg out into large, colorless, ball-like vesicles 

 always poor in cytoplasm. In cuttings of Sambucus I found 

 that the thick-walled cells of the collenchsmia strands also 

 participated in the .hypertrophy. In the cases I have studied 

 the surface gtowth of the membrazie was restricted to the thin- 

 walled parts. This seems to hold good also for the coll enchyma 

 cells of the Ricinus stalk which Massart-^ saw hypertrophy 

 strongly after injury. I have never been able to observe in 

 cuttings of this kind that coll enchyma cells can be stimulated 

 to cell-division after injury, in other plants the bark cells 

 are chiefly elongated, furnishing colorless sacs radially ar- 

 ranged. A VBDTy luxuriant growth ^akes place, for instance, 

 on the cut surfaces of Syringa cuttings, the hypertrophied 

 ban cells of which are often from tv/o to ten times as long 

 as they are wide. / 



As yet- I have never observed that the bark cells of 

 woody plants, hypertrophied after injury, had undergone me- 

 taplastic, changes. In future investigations exceptions to 

 this rule may possibly be found. Combinations of abnormal 

 gjtrowth with metaplastic changes of the cell- character are 

 already laown in callus hsrper trophies of leaves and in those 

 mentioned in the following lines. 



f^^) Callus hypertrophies of an unusual kind are 



illustrated hj the tyloses which, as is well knovm, 

 are produced' by the outgrowth of wood-parenchyma cells 

 after injury, thus filling the liimina of the ducts. 

 Since only a small section of the parenchsnna cell wall 

 takes part in the surface growth during the production 

 of tyloses, it is allowable to compare these with the 

 hypertrophied products of the coMenchyma cells men- 

 tioned above. However, the tyloses differ 'so essen- 

 tially from all other callus-hypertrophies, histolo- 

 gically and etiologically, that they may be reserved 

 for special consideration in the next section. 



Leaves 



Leaves of the monocotyledons and dicotyledons.- espec- 

 ially of the former',- often form after injury very volumin- 

 ous hypertrophies, from the exposed layers of their mesophyll. 

 Pig. E5 shows hov/ far the growing cells may In this way ex- 

 ceed their original 'v'olume. Thus the character of the cells 

 is changed generally, in the usual v;ay, by the disappearance 

 of its cytoplasmic content and the disintegration of the chlo- 

 rophyll, v/ithout the development of new anatomical characters. 

 An exception worth noticing is made by Cattleya, doubtless 



also by still, other orchids. The cells on the edge of the 

 wound, illustrated in figure 25, form reticulated thickenings 



1. La ci68,trisation chez 1* vegetaux. Mem. cour, etc* 

 Acad. Belgique, 1898, T. LVII, p. 44* 



