CONTRIBUTIONS FROM THE WISLEY LABORATORY. 



43 



21 b). They then apparently spread gradually inwards, through the 

 stages shown in figs. 20 c, 21 d, until the condition shown in figs. 21 c, 

 21 F was reached. 



A soft and rapid black rot of the kind characteristic of the " ring- 

 disease " form of black-leg does not obtain. Even the worst tubers 

 were found to be capable of producing disease-free plants. Small 

 isolated greyish-brown areas occurred in the case of the variety 

 ' Northern Star, ' from East Lothian, already mentioned, but in some 

 tubers the principal vascular bundles were of a brownish colour, not 

 black, as in the ' British Queen. ' The disease in this instance actually 

 increased during the season. In one specimen a brown zone developed 

 about the principal bundles, and by April small areas of tissue within 

 this zone had disintegrated. The greater portion of the tuber, however, 

 remained sound. Streak-disease occurred in this sample, but in this 

 case it did not actually occur together with bruise in the same tuber. 

 The two diseases frequently do occur together; nevertheless, they are 

 quite distinct forms, and should not be confused and treated as one and 

 the same. 



In typical examples of bruise, a microscopic examination shows that 

 the speckled appearance is due to the disease not spreading uniformly 

 but in irregular lines, so that in early stages series of affected and 

 unaffected cells are interwoven. In sections of fresh material an inter- 

 esting series can be traced from normal to diseased cells. 



In a short paper''' read before the University of Durham Philo- 

 sophical Society, the writer explained that with careful manipulation 

 the protoplasm in the cells of potato tubers could be observed to move. 

 This movement is most easily seen in cells which are performing some 

 kind of work. If a potato be exposed to direct light it becomes green 

 after a short time, owing to the development of chlorophyll in cells 

 which under normal circumstances function for the storage of starch. 

 Correlated with the appearance of chlorophyll the starch grains diminish 

 in size until only a fragment of the original grain remains. If these 

 cells be examined protoplasmic movement may be seen. It may also be 

 observed, but with greater difficulty, in the starch parenchyma, so that 

 it becomes possible to distinguish living from dead cells. This can 

 actually be done on the border-line between the healthy and unhealthy 

 tissues where series of living and dead cells are interwoven. 



Both the living and dead cells are stored with starch, so that the 

 earliest post-mortem change, which consists only of an altered appear- 

 ance of the protoplasm due to coagulation, is with difficulty visible 

 owing to the fact that the protoplasm itself is obscured by the starch 

 grains. 



The protoplasm in the cells adjacent to the coagulated ones 

 appears to be permeated with a dark insoluble matter. It becomes 

 altered more and more until finally the original structure is 

 entirely obscured. There is no easily observed alteration of the cell- 

 wall in early stages of the disease. The altered protoplasmic mem- 



* A. S. HoRNE, Proc. Univ. of Durham Philos. Soc. 



