m 



Investigations of young fruit show, however, that in each variety of 

 pear in normal development aggregations of coarse- walled schlerenchymatous 

 cells are always present unequally distributed. These stone cells are in fact 

 an anatomical characteristic differentiating pears and apples^ Therefore, it 

 is not the occurrence of the stone cells but only the greater thickness of the 

 walls already formed which is the result of the drought. In many varieties 

 they remain relatively thin-walled. To this should be added that their con- 

 nection with the surrounding tissue is tougher and closer in dry years. 



In the so-called stoniness of pears, only the increased wall-thickening- 

 of the normally deposited schlerenchy- 

 ma cell centres is concerned and there- 

 fore no increase of the elements, while 

 we find in Lithiasis an accumulation 

 of stone cell elements produced subse- 

 quently by cell increase. These finally 

 may also extend over the surface of 

 the fruit and then form light brown 

 circular specks, either equally distrib- 

 uted or clustered on the sunny side or 

 even map-like etchings due to the run- 

 ning together of the specks (Fig. 19), 

 the upper surface of w^hich shows a 

 crumbly construction. Not infrequently 

 the same varieties of pear suffer 

 also from Fusicladium (see Vol. II). 

 Nevertheless, the Lithiasis specks may 

 be easily distinguished from the smooth, 

 usually blackened, fungous specks, be- 

 cause of their crumbly constitution and 

 the raised edges of the wound. 



So far as observations have shown 

 as yet, only certain varieties suffer 

 from Lithiasis. Many, in fact, form 

 predominantly roundish specks, while 

 in others usually zigzag gapping cracks 

 are produced. Stone masses are not 



always depressed, often they occur on the upper surface as pale cork-colored 

 cushions. 



An entirely normal construction may be found in the healthy parts of 

 the pear attacked by the stone disease; i. e., underneath the small celled, not 



Fig-. 



19. Pear diseased with 

 Lithiasis. (Orig.) 



1 Turpin, Memoire sur la difference qu'offrent les tissus cellulaires de la pomme 

 et de la poire etc. Paris. Compt. rend. 1838, I, pp. 711 ff. 



- The substance, of which the stratified thickened walls of the stone cells con- 

 sist, has received the name of glycodrupose from Erdmann*. He used this name 

 because he thought that the chemical composition of these cells is the same as that 

 of the tissue of stones of plums and cherries (Drupaceae). The substance, decom- 

 posed by moderately concentrated hydrochloric acid, gives half its weight in grape 

 sugar in solution. The half remaining undissolved is called drupose; when boiled 



