308 Ward. — On Relations between Host and Parasite 
shrinkage in breadth due to the action of the reagents em- 
ployed in preserving and mounting. The danger is a real 
one, because the contraction of the motor-cells under certain 
circumstances may vitiate the measurements by drawing 
together the whole upper surface of the leaf. I found, how- 
ever, that with the properly hardened material relaxed in 
glycerine jelly and measured at once, this danger was reduced 
to a minimum, and the measurements compare very accurately 
with those made on fresh specimens. Nevertheless, I took 
the precaution to institute check measurements (see columns 
3 and 4) of the rib-intervals throughout by two different 
methods, and it will be noticed how closely the results set 
forth in column 1 (measurement with a lens direct) usually 
agree with the totals in columns 3 and 4. Moreover, it should 
be noted that exactly the same treatment throughout was 
accorded to all the species. 
In measuring the intervals, in column 3 the micrometer 
scale 1 was made to travel from the axis of one rib to that 
of the next, or (column 4) from the centre of one vascular 
bundle to that of the next. It will be noticed that the 
distance from rib to rib is often greater on one side of the 
mid-rib than on the other. This inequality in the breadth 
of the two halves of the lamina of grass leaves has often been 
observed, but, so far as I am aware, no explanation has been 
given of it. It appears to be due to the fact that the smaller 
half-lamina was the innermost in the involuted state of the 
leaf in the bud, and thus had less room for lateral extension, 
its growth being hindered by the pressure of the surrounding 
parts — the enveloping outer half-lamina and sheath — between 
which it is trapped, as it were. The same mechanical pressures 
help to arrest the longitudinal growth also, and so induce 
the twist 2 in the lamina by rendering one edge shorter than the 
other. 
1 I used a Zeiss ‘ Mess-ocular ’ with travelling scale, and a mechanical stage. 
2 This twisting of adult grass leaves has often been noticed. See an interesting 
paper by Max Wichura in Scientific Memoirs , Parts III and IV, 1853, p. 262, 
where, however, the author failed to give any satisfactory explanation of the 
phenomenon. 
