jl 
34 PROF. JULIUS MACLEOD ON TEN 
the first period, including the leaves of the lower part of the stem, is suddenly 
interrupted at a certain level, and followed by a second period which begins 
with very small leaves and goes on regularly to the summit. The limit 
between both periods is distinctly indicated by a breach of continuity in the 
gradation of the length of the successive leaves. 
When two periods exist, only the ssconp PERIOD (which ends at the 
summit of the stem) is taken into account. 
$24. PREPARATION OF THE LEAVES.—Herbarium specimens and alcohol 
material, having been placed in hot water for a few minutes, may be used as 
well as fresh objects. All the leaves are separated from the stem, proceeding 
from the base towards the summit, under the simple microscope, by means 
of a scalpel. The leaves are placed in their order of succession (their back 
upward) on slides, in a very small quantity of water (just enough to prevent 
desiccation). They are then covered, the underside of each cover-glass 
being moistened with diluted glycerine. If a leaf is brought at once in 
(even diluted) glycerine, it may curl up before it is possible to place the 
cover-glass over it. 
$25. LENGTH OF THE LEAF.—At the base of a leaf separated from the stem 
one finds easily a more or less irregular curved line e, corresponding to the 
upper limit of the surface of insertion of the 
leaf. I take as inferior limit of the leaf the 
point 7, which is in the centre of the surface 
limited by the curve ¢ and the transversal 
straight line bb, the latter being the span of 
the arch c. 
Following the nerve, I measure under the 
mieroscope the length of the leaf from the 
extreme summit to ¿ As it is impossible to 
use a high power according to the dimen- 
sions of the object (I used obj. 3 Leitz), and 
as the position of 7 is not strictly determin- 
able, a positive or negative error of 30 u 
(0:03 mm.) is possible. 
Fic. 5.— Lower part of a leaf 
separated from the stem. 
See text. n, nerve. 
$26. BREADTH OF THE LEAF. PLACE OF THE GREATEST BREADTH.—In 
almost all the leaves the place of the greatest breadth is easily found, the 
forms represented in fig. 4, 7 and 4 (page 33), being the commonest. In 
some leaves the greatest breadth extends over a certain length, the margins 
being parallel (fig. 4, 2, 3) : here we agree to take the upper limit of the 
parallel portion (indicated by B in fig. 4, 2, 3) as the place of the greatest 
breadth. Among the lowest leaves a form similar to fig. 4, 5, sometimes 
