"Predisposition" and "Immunity" in Plants. 319 



i.e. the first green leaf, — grown under the same conditions in every 

 case, and several methods were adopted for obtaining the measure- 

 ments, so that one should check the other. For instance, in count- 

 ing the stomata and hairs it will be noticed that I not only obtained 

 averages as described below, but in the case of every species I also 

 plotted a chart of a square millimetre and so obtained a direct 

 count (see Table IV.), and other checks are described in the 

 sequel. 



It is of course impossible to suppose that the numbers ob- 

 tained can be absolutely correct, but the tabulated results show 

 such agreements amongst themselves, for the most part, that I am 

 disposed to regard them as more accurate than is usually the case 

 in such approximate determinations. It is unnecessary to dwell 

 on the laborious nature of the determinations, but of course this 

 limits the number of countings. 



I append an account of that part of the analysis which concerns 

 the stomata, and in Table V. the summary of results so far as the 

 stomata are concerned. 



Very little work appears to have been done on the dimensions, 

 numbers and distribution of the stomata of grasses, and none, so 

 far as I have been able to discover, on those of Bromus or on the 

 seedlings of any genus. 



It has been known since the time of Weiss 1 and even before 

 him, that the stomata of grasses are arranged in longitudinal rows 

 flanking the ridges, and that in some genera or species they occur 

 only on one side, usually the upper, and in others on both sides of 

 the leaf. In all the Bromes I have examined 2 they occur on both 

 sides, and are more numerous on the upper than on the lower face 

 of the lamina. Owing to their peculiar distribution on the flanks 

 of the ridges it is not always easy to observe and count the 

 stomata by direct observation of the faces of the leaf, since many 

 of the stomata are seen obliquely from the side or are even more 

 or less withdrawn from observation (unless great care is exercised) 

 owing to the contractile action of the motor cells, which causes 

 them to sink into the flanks of the narrow furrows as the leaf rolls 

 inwards. 



It seemed that, since infection occurs by means of the stomata, 

 an accurate knowledge of their sizes and numbers was, then, an 

 imperative necessity, in order to test whether any relation existed 

 between these factors and the susceptibility to infection of the 

 species. 



Consequently, I undertook the very laborious task of count- 

 ing the stomata per square unit of surface of the leaf, both above 



1 Prhigsli. Jahrb. B. iv. 



2 With one exception, a species which requires further investigation in other 

 respects. 



