20 



extended at least to the soft bast, that is, nearly halfway through the 

 leaf. The proteid sheath secreted by the insect while inserting its 

 puncturing- bristles was well stained with carbol fuchsin and gentian 

 violet by allowing the sections attached to the slide to stand in the 

 stain at a temperature of 55° C. for two hours. A slight and transient 

 staining is thus obtained in fifteen minutes. 



These sections showed, as Biisgen^ pointed out in the case of many 

 other plants, that the puncturing apparatus does not lacerate the 



cells, but passes between them, curv- 

 ing around obstructions and following 

 the middle lamella as shown in the fig- 

 ures cited, the gelatinous sheath being 

 secreted from the mouth parts of the 

 insect as a support for the fine bristles 

 as thev wedge their wav between the 

 cells. The tip of the puncturing ap- 

 paratus stops in the region of the elon- 

 gated soft bast cells, which it punctures 

 and from which the proteid contents 

 are removed. Sugar is also sucked 

 out of the cells in this region, passing 

 through the alimentary canal and ap- 

 pearing on the plant as honeydew. It 

 is probable that part of the sugar is 

 used as food b}^ the insect, but the 

 larger part is excreted, while most of 

 the proteid substances are retained and 

 evidently constitute for the insect the 

 principal food element extracted from 

 the tissues. 



INFECTION AND COLONIZATION EXPERI- 

 MENTS IN 1898. 



Fig. 5. — Cross section oi a young carnation 

 leaf showing an aphid's sucking appa- 

 ratus in the tissues. The tip of the beak is 

 in the soft bast of the vascular bundle. 

 The first puncture passed to the right be- 

 tween the two bundles, and after the with- 

 drawal of the beak from this region the 

 proteid supporting sheath secreted by the 

 insect remained between the cells. No 

 apparent change has yet taken place in 

 the cells. (Drawn with Zeiss camera lu- 

 cida, X 225 diameters.) 



The experiments described proved 

 pretty conclusively that the disease in 

 question must be attributed to the punctures of insects and mites, 

 it having been shown definitel}^ that plants kept free from these 

 animal organisms remained perfectly free from the disease, and that 

 when they were present it invariably developed. In order to settle 

 the matter beyond any possible doubt, colonization experiments with 

 insects were carried on under perfectl}'^ antiseptic conditions and 

 inoculation experiments made with bacteria upon plants which had 

 been kept absolutely free from puncturing insects and mites. 



1 Biisgen, M., Der Honigtau. Biologische Studien an Pflanzen und Pflanzenliiu- 

 sen, Bes. Abdr. a. d. Jen. Zeitsch. fiir Natiirwis., Bd. XXV, N. F. Bd. XVIII. 



