50 MISSOURI BOTANICAL GARDEN. 
ing of amygdaloides might seem a barrier to such a union, 
but when closely noted it is found the variation in this 
respect is greater than formerly suspected; besides, the 
union need not be directly with the pure form of amygda- 
lotdes but with its hybrid amygdaloides X nigra, flowering 
later. 
Whether or not &. longipes should be divided into sevy- 
eral forms or varieties is a question, owing to insufficient 
accumulation of material, not yet determinable. In view 
of what we know at the present day of the instability of 
some of our species of willows, it does not seem to fill the 
present demands of the scientific idea, to set up a number of 
forms from a limited number of herbarium specimens, and, 
besides, incomplete as we often find them. Without care- 
ful field observations, such work must inevitably prove de- 
fective. The plant must be seen in its habitat and in 
quantity before a just conception of all its characters can 
be formed. In this way, the supposedly good form will 
often vanish, to be supplanted by easily recognizable vari- 
ations. 
S. longipes and S. nigra afford a most remarkable and 
extremely interesting example of differentiation, retaining 
subtle resemblances which cause one sometimes to feel hes- 
itation in regarding them as distinct. Thus, likeness of 
the veining, the general forms of the leaves, including 
serration, the pubescence, the gray or brown bark of the 
branchlets, the similar roundish lenticels, the resemblance 
of the scales, stipules, stamens, capsules and stigmas, con- 
tinuation of the flowering stem, attacks of the leaf gall- 
mite, all these and other characteristics, with their shades 
of difference, besides, constantly remind the observer of 
their very near relationship. In view of this, perhaps the 
most astonishing fact is that though covering the same 
geographical range over the southern portion of our coun- 
try, they should not have become fused into one, or inex- 
tricably confused, as is the case with cordata and sericea 
where found coincident. And yet, this very persistence of 
8 
