254 



the ooourrence of the absence of a. gall-plastein^, Fe find at 

 times v/itih the aarae kind of cell-division, the products of 

 which may he called gall plastein, a production of highly dif- 

 ferentiated galls and at tiin«a one of simple, homogeneous tis- 

 sue exorescenoGS. The quality of the stimulus Is the decisive 

 factor, 



I again call attention here to the bark gall of Ohermos 

 fagl, described by Hartlg, which is likewise produced by abun- 

 dunt tangential cell-divisions, as also the richly differentia- 

 ted Banister la gall. Illustrated in Figure 9S. The.fornBr 

 arises partly from bark tissue which Is several years did and 

 thus furnishes proof that even mature tissue IS still fit to 

 produce galls. At any rate the Chernee gall here mentioned con- 

 sists of the simplest, undifferentiated tissue, corresponding 

 histologically to the callus excrescences produced after Injury, 



It must also be emphasized here, that gall formr.tlons are 

 but little suited for a treatment of the question as to the 

 distinct capacity of young and old tissues for reaction, sinod, 

 for the present, in a decision as to the possibilities of the 

 plant, we are dependent upon material present In nature end 

 since no experlmentajC treatment of the question has yet been 

 successful, Slncd the gall insects always deposit their virus 

 In the same kind of tissues, we can not find out how other tis- 

 sues would act under similar treatment, i, ©, after infection 

 (296) v/ith the same poisons. The existence of any difference In prin- 

 ciple between tissues of different ages, so far as their behav- 

 ior toward chemical substances it concerned, is but little 

 probable, since the cells even of permanent tissue several years 

 old, like those of young tissue materlal,oan be incited to sim- 

 ilar or the same reaction to stimuli^ by very different r 

 stlmiuli of some other kind. 



Often cells of the permanent tissue are Inoited to abnormal 

 growth and division by injury - instances of which are furnlshftd 

 by tyloses, and callus excrescences of the bark and pith, I 

 may also call attention here to ^.dventitious sprouts produced 

 after Injury en mature Begonia leaves. The abH.lty to "restitu4©7T 

 tissue which has been lost by mutilation is ptssessed particular- 

 ly by young tissues (compare chapter 1)« 



Kny has recently proved in Impatlens and others that the 

 cells 'of the permanent tissue can also be incited to division 

 ^y roe ohaiiic al fa cto r s . 



We find that hyperhydric phenomena of growth, occuA in 

 young bark cells as well as in those which are several years 

 o3.d, (Compare Chaptej? IV, S). 



^ Appel calls attention to the fact, "that gall formations 

 can be produced in exactly the same stage ©n the same plant and 

 yet their raorphologdoal significance may be very unequal. Thus 

 the leaf louse ?hyllapsis fagl produces simple warping and 

 crimpling cf the leaves, the Hormomyia species, on the contrary, 

 structures of comparatively high differentiation. Both stimuli 

 however, act in the same period, if not exactly at the same time; 

 i, », at the time of leaf distension. The difference consists 

 in this, thr.t the stimulus of the leaf ^.ice encounters In this 

 case mature tissue, which can not be trans;^3r;ged into one re- 

 sembling plastein, the Stimulus of the HorraomB^a, however. Is 

 able to degelbp embryonic tissue, from which jj,©^ tissue struc- 

 tures can be differentiated, (loo. «it.). 



