780 



SCIENCE 



[N. S. Vol. XLI. No. 1065 



this substance is different in seedlings of 

 different kinds. Sorauer found resistance of 

 different carnations to be due to thickness 

 of the cuticle. It might be caused also by 

 the wax layer, which is present in Gram- 

 inea, carnations, and other plants. 



In his experience the writer found that 

 the wax layer influences the attack of 

 Coniothyrium on raspberries. In a large 

 horticultural establishment varieties which 

 were covered by a thick blue wax layer 

 were free from this disease, while other 

 varieties were completely killed. The wax 

 layer may exert its influence in different 

 ways, that is it may prevent direct pene- 

 tration by the hyphse or it may act indi- 

 rectly by causing the moisture to run off 

 the plant. This was observed by the 

 writer in making sprayings with Bor- 

 deaux mixture. In the case of plants cov- 

 ered with the wax layer the mixture ran 

 off quickly and left no moisture. Con- 

 flicting results have been obtained from 

 observations of Gloeosporium venetum on 

 raspberries on the fruit farm of the Uni- 

 versity of Minnesota. There is no differ- 

 ence between raspberries with wax and 

 without wax. Glceosporiii/m venetum, how- 

 ever, has very sticky conidia and is held 

 by the wax layer, while Coniothyrium 

 spores are washed away. 



The hairs on the surface also play a 

 part in this connection. Their unfavor- 

 able influence in the case of potato late 

 blight has already been mentioned. A 

 very interesting case of hair-like struc- 

 tures is found in the pea family. In some 

 varieties the seeds are imbedded in a woolly 

 outgrowth of the inner epidermis of the 

 pod. Frequently when pods are infected 

 with Ascochyta pisi the fungus penetrates 

 into the interior. In varieties without these 

 hairs the seeds are infected only when they 

 are directly in contact with an infected 

 spot of the pod. But when the interior is 



covered with the woolly outgrowth the 

 fungus grows as in a culture medium and 

 infects every seed. 



The cork, which is without doubt a pro- 

 tecting tissue, is a deflnite kind of epi- 

 dermis. The writer has never seen 

 branches of cork elms attacked by fungi, 

 but the common elm is subject to the at- 

 tacks of several species. In the . case of 

 the potato the cork layer has the greatest 

 significance. 



The causes of the protecting action of the 

 cork, however, may be different. Certain 

 fungi are able to penetrate this cork layer, 

 such as Phytophthora, and probably 

 Fusarium and Spondylocadium. But the 

 last-named fungus is able to penetrate only 

 the very outermost layers of the potato, 

 where it forms mycelium and sclerotia 

 normally. Whenever it grows into the 

 tissues below it must use the channels al- 

 ready opened by other fungi which may 

 happen to be present. Thick cork layers 

 seem to be impenetrable for Phytophthora 

 and Fusarium. The questions involved are 

 very difficult to solve, because it is hardly 

 possible to judge whether a cork layer is 

 intact or not. 



As small wounds occur very generally, 

 the rapidity with which wound cork is 

 formed is possibly of more importance 

 than the absolute thickness of the cork 

 layer. In the course of work with black 

 leg of the potato the writer was able to 

 study this question. It is easy to cure a 

 bacterial infection artificially. The po- 

 tato is able to close a wound within a short 

 time by the formation of cork. When the 

 growth of bacteria is diminished by low 

 temperature or drought the potato closes 

 wounds more rapidly than the bacteria 

 can penetrate. The ability to form wound 

 cork varies in different varieties of pota- 

 toes. Some varieties begin cork forma- 

 tion within six hours after the wound is 



