H. WORMALD 185 



(1) 111 every case, with one exception, viz. D 22 [=325; 32G], 

 the range of variation (difference between minimum and maximum) 

 is greatest for the lower leaves; this was found to be the case also for 

 the great maiority of the rest of the plants examined. 



(2) With the exception of Z 12 [= 292; 293] the glands are more 

 numerous on the lower leaves than on the upper, as shown by the 

 average of the 18 countings in each instance; this condition again 

 obtained throughout the rest of the garden except in seven plants, 

 where, however, the difference was not considerable as is often the 

 case when the difference is in the other direction as in 168 [= 167 ; 

 169] of the above table. 



It is advisable therefore when making observations in this connec- 

 tion, to select leaves at about the same level, which, for preference, 

 should be a fairly high one. 



Of all the plants examined the one with the highest average is the 

 one appearing last in the table, viz. 168 [= 167; 169], with an average 

 of 82; others with numerous glands are A 13 [=315; 316] as shown 

 in the table, and H 22 [= 26] which has given 73 as the average of 

 24 countings. The Oregon ,^ hops too have shown a high average, 

 viz. 67, the maximum and minimum numbers obtained being 117 

 and 50 respectively. 



Of those plants producing few glands the most striking are 



F 2 [= 309; 310] shown in the table to have an average of 31. 

 269 [= 297] min. 18, max. 37, av. 28 (for 24 countings). 

 67 mill. 16, max. 31, av. 23 (for 12 countings). 



The plant giving the lowest average of all, viz. 16, is K 1, a seedling 

 obtained by crossing a Canterbury Whitebine with an Oregon cj. 



Size of the glands. The glands are easily seen if a fresh leaf or 

 portion of one is placed with the lower surface upwards on the stage of 

 a microscope and examined by the aid of a | in. objective. It is 

 preferable to use reflected light only, for then the glistening yellow glands 

 contrast well with the dark green of the leaf surface. The diameter 

 of the glands was measured by means of an eyepiece micrometer; 

 during 1912 the ordinary type'^ of eyepiece micrometer was employed, 

 but during the two succeeding years the Leitz's Stufen-Mikrometer 

 was found to be not only much more convenient, but also the divisions 

 of the scale were more easily seen against the dark background of the 

 leaf. In this way it was found possible to ascertain rapidly the diameter 



^ I.e. the type in wliirli a glass circle with scale is dropped into the eyepiece. 



13—2 



