PARASITIC ROSE CANKER. 



227 



fungus produces its fructification, in this case the black round conceptacles 

 mentioned before. These conceptacles are formed from the interlacing 

 mycelium and when ripe become hardened and push their heads through 

 the epidermis for the purpose of shedding their spores. 



We may now proceed to consider the development of the cankerous 

 spots. What follows now is practically natural devel pment. A normal 

 and healthy branch when growing expands all round in a uniform 

 manner, the bark giving way by means of its elasticity increased by its 

 more or less pn nounced perforation by lenticels. But when portions of 

 the bark are killed, as in the case under observation, the natural result 

 will be that where growth is hindered by dead tissues, cracks arise. In 

 this manner the longitudinal cracks in the sho >ts which are shown in 

 fig. 34, k, result. At l we have one of the more open cracks enlarged ; 

 with a cross section through the injured p rtion at m. Here only the 

 split and gaping bark at the upper portion is to be seen. To understand 

 the further development as shown at n, it is well to call to mind the 

 pn cess which takes place in a plant endeavouring to heal a wound as in 

 cases where branches are cut away from a tree. In a short time from 

 underneath the old bark, a new growth of cells, or callus, arises all round 

 the wound, which in due course and under normal conditions is finally 

 quite covered by the meeting of the over-growing callus in the centre. 

 Practically the same thing occurs before a cutting produces roots. The 

 growth seen to rise from the sound edsres of the wound figured at n is 

 really of the same nature as this well-known callus. At o a section 

 through the diseised part of n is shown illustrating the manner in which 

 the callus protrudes from underneath the bark. If the callus would grow 

 completely over the wound and unite in the centre the wound would be 

 healed, but this the fungus prevents. The pirasite finds in the callus no 

 resisting epidermis and needs no wound to enable it to effect an entrance 

 into the soft tissue. It makes short work of these cells and the host tries 

 to repair the damage wrought by the fungus by continuing the formation 

 of healing cells. The struggle between the fungus, which kills the tissues, 

 and the host, which by making new c illus endeavours to heal the wound, 

 goes on and on, and at last ends in the producti -n of badly cankered spots 

 as shown at p. A section through stem and growth is shown in q. The 

 woody part is still intact, and remnants of the baik are adhering to the 

 right and a small portion of the old bark covering the stem is balanced on 

 the growth towards the extreme left. All along the edge the black fungus 

 bodies surrounded by darker tissues which are filled by mycelium are to 

 be seen. At r a portion of the callus growth is magnified showing five 

 fruiting conceptacles and the uniform cell chains comp sing the growth. 

 The absence of any protective covering is also to be noticed, although to 

 the left a small fragment of the old bark still adheres. 



Thus far the life of the host is not much impaired, the portions above 

 the place of injury still deriving sufficient nourishment through the 

 sap-conducting vessels of the uninjured portion. Death will only result 

 if a cankerous growth extends all round the stem and thus separates 

 the upper part from communication with the root. How this end is 

 reached is easily explained. We have seen that the callus cells are 

 unprotected and possess thin walls ; so that in the case of even a very 



q 5i 



