38 PROF. JULIUS MACLEOD ON TEN 
§ 32. NUMBER OF MARGINAL TEETH.—No explanation is needed if all the 
teeth are simple. If certain teeth are in groups of 2 (even of 3) each group 
is brought into account as 1. ‘Teeth which are situated below the base of the 
leaf (see 7 in fig. 5, page 34) are excluded *. 
It is sometimes difficult to say whether a given marginal cell ought to be 
considered as a tooth or not. I consider as a tooth each marginal cell, the 
distal extremity of which projects beyond the margin by at least one-half of 
its breadth (see fig. 7). 
Fre. 7.—Rudimentary teeth. m, margin; a and o are considered as teeth ; 
c is not taken into account as a tooth. 
The figure of a leaf is the total number of teeth at the right and the left 
margin. 
I estimate the possible positive or negative error at 1 tooth, if all the 
teeth are simple. If double or triple teeth exist, the error may reach 3. 
§ 33. NUMBER OF DOUBLE (OR TRIPLE) MARGINAL TEETH.—In several species 
double or even triple teeth exist. These are counted separately. Each 
group (of 2 or 3 teeth) is taken into account as 1 unit. Figure of the leaf 
noted as in $32. Possible positive or negative error, 2. 
§ 34. TEETH ON THE NERVE AT THE BACK OF THE L&AF.—(Careful obser- 
vation is needed for the counting of these teeth. A group of 2 teeth 
(situated at the same level—this rarely occurs) js brought into account as 
l unit, Error: the observed number may be too small by 1 unit; a positive 
error is almost impossible. 
$35. LENGTH OF THE MIDRIB (reaching the summit or not).—The length of 
the nerve is very variable. A measurement of this length is difficult, because 
* Such teeth occur in certain decurrent leaves: for instance, in some leaves of Mnium 
undulatum. 
