426 
CARL S. HOAR 
uniseriate. This is not a feature characteristic of the whole group; 
but is rather the exception. Take, for example, our common form, 
Alnus incana L. Here the entirely uniseriate rays contrast with 
apparent aggregations of rays. In the latter the slender component 
rays are separated from one another by fibers only; the vessels char- 
acteristic of the remaining wood structure being conspicuous by their 
absence. Alnus rugosa Du Roi shows a condition very similar to 
that of Alnus incana and therefore needs no special description. 
In the case of Alnus japonica a somewhat different situation is 
present. In figure 3 is shown part of the woody cyHnder of a three- 
year-old branch of this species. Vertically in the center is seen the 
aggregation of rays corresponding to a leaf-trace, terminated below 
by the trace itself and subtending the leaf-gap. In the aggregation 
of rays, vessels are clearly absent. On either side of this central con- 
gery of rays lies the wood which has the ordinary structure except 
where smaller aggregate rays are present. The latter are the dwindling 
remnants of foliar aggregations belonging to nodes higher or lower in 
the stem. 
Figure 4 is a tangential view under about the same magnification, 
taken from the stem of the same species. In the center near the top 
may be seen the round cluster of cells which makes up the leaf-trace 
and which pass out horizontally to the axis of the tree. Extending for 
a short distance above this and for a much greater distance below, a 
noticeably larger clustering of elements may be observed, differing from 
the surrounding tissue by the absence of vessels. Careful examination 
shows it to consist of groups of rays (one to three cells in diameter) 
and separated by tracheids only. In other words, it is an aggregate 
ray formed about the leaf-trace for the more abundant storage of food. 
Various other alders show this tendency to aggregate still further 
carried out. In Alnus rubra the condition is quite like that of Alnus 
japonica with the rays gathered together and becoming more or less 
multiseriate. Still further compounding appears in Alnus maritima 
Marsh. It may be inferred from the above description that the alders 
illustrate as regards ray organization a relatively low condition. 
Interesting situations are presented by Alnus mollis Fernald, a 
species of northern range commonly found throughout eastern Canada 
and the Eastern States, and only reaching southward along certain 
streams which have brought the seeds down in their current. In these 
southern limits of distribution, the species is much smaller in size than 
