ROSE CANKER AND ITS CONTROL. 25 



been seen when the material was fixed. In some cases the affected cells 

 extended no farther than 5 or 6 rows below the stomate. 



There does not seem to be any reasonable doubt that the stomates 

 serve as infection courts, and that the little round lesions on the smooth 

 stems are largely the result of these stomatal infections. 



The Mycelium in the Host Tissues. 

 In order to follow the course of the mj'celium after it has entered the 

 rose stem, and to determine its effect on the host tissues, cankers in every 

 stage of development, from that where they are not yet visible to the 

 naked eye up to the old, fully developed lesion, were sectioned, stained 

 and studied. 



Method. — The mycelium is very difficult to follow in unstained sections, but 

 after some experimenting a simple taethod of treatment was found by which the 

 mycelium could be very distinctly differentiated in the host cells. Cankers were 

 fixed in Gilson's fluid, dehydrated gradually, and cut with a slide microtome from 

 95 per cent, alcohol, i The sections were then stained one minute in a saturated 

 solution of safranin in 95 per cent, alcohol, excess safranin removed by trans- 

 ferring to 95 per cent, alcohol for one minute, stained one minute in 1 per cent, 

 gentian violet in clove oil, and cleared in clove oil, the oil washed out with xylol 

 and the sections mounted in balsam. This method is very rapid and any number 

 of sections can be stained at one time. 



Before describing the behavior of the mycelium in the tissues it will 

 first be necessary to review briefly the structure of a normal rose stem. 

 Fig. 9 represents a cross-section of a stem of about the age when cankers 

 are most frequent. 



Normal Structure of the Stem. — On cutting through a rose stem with 

 a knife, one very readily notices that it is composed of three distinct 

 parts, (1) a rather succulent outer cylinder of bark, (2) a central soft 

 white pith, and (3) a hard cylinder of wood between the two. The cell 

 elements which occur in each of these will be enumerated in order, be- 

 ginning with the outside. 



First, the stem is covered with a smooth, thin, waterproof coat, — the 

 cuticle. Just beneath this is the one layer of rather flat cells composing 

 the epidermis. Next in order are three or four layers of cells with heavy 

 walls and no intercellular spaces. This is the collenchyma. The cuticle, 

 epidermis and collenchyma form an air-tight, water-tight covering of 

 the stem, uninterrupted except by the stomates. These microscopic 

 breathing pores, which are not so numerous on the stem as in the leaves, 

 are guarded and strengthened on either side by crescent-shaped pro- 

 jecting cells. The structure of the stomate can best be understood by 

 reference to the figure. It will be noticed that there is a free passage 

 between the guard cells into the stomatal cavity beneath, and from here 

 to the loose, thin-walled cells of the next underlying tissue, the chloren- 



1 Very small cankers were imbedded in paraffin, sectioned and stained in the usual way; but 

 for larger cankers this was found to be unnecessary, and a long and tedious process. 



