1913] HEMENWAY—SIEVE-TUBE 2390 
Species of over 140 genera, belonging to more than 60 families, 
have been studied. The sieve-tubes found in these different 
species, for the sake of convenience, may be grouped under three 
types, though perhaps most of them will come naturally between 
the first and second or second and third types. 
The first type is like that found in Pinus. Here the lateral 
sieve-plates are the same as the terminal ones, and the end walls 
are very oblique, extending from one-fourth to one-half the length 
of the sieve-tube. Text fig. 1 shows one-third of a sieve-tube of 
Juglans nigra as séen in tangential view. The lateral sieve-plates 
are seen in face view on the tangential wall, while the terminal 
sieve-plates are seen in transverse section. The lateral sieve-plates 
on the tangential walls of the sieve-tubes in Juglans are usually 
more irregular and thinner than those on the terminal or radial 
walls. 
The second type is like the first except that the lateral sieve- 
plates are less well developed, and the end walls are less oblique, 
and have 2-10 sieve-plates each. This type may be illustrated 
by Vitis (text fig. 2). The figure shows one-half of a sieve-tube in 
tangential section. The end wall here has 7 sieve-plates covered 
with callus. Poorly developed sieve-plates are shown in face 
view on the tangential wall. On the left is a companion cell 
related to the sieve-tube by fine pits. 
The third type has end walls that are nearly at right angles 
to the side walls, and has only one sieve-plate to each end wall. 
The sieve-tubes of Lactuca scariola illustrate this type (text fig. 3). 
This sieve-tube, though shown in full length view, was drawn to 
the same scale as the other text figures. The three lattices on the 
left relate it to another sieve-tube. 
The species studied may be grouped under these types as follows: 
FIRST TYPE® 
Alnus incana, A. rugosa, A. oregana, Betula alba, B. lenta, B. lutea, 
Banksia Menziesii, Carya alba, C. ovata, Castanea dentata, Castanopsis 
chrysophylla, Casuarina Fraseriana, C. equisetifolia, Corylus americana, 
6 The nomenclature of Gray’s Manual (Ed. 7) is followed as far as applicable; 
the names of plants indigenous to Oregon are those used in Howett’s Flora of 
Northwest America. 
