ENGELMAMN OAKS OF THE UNITED STATES. 379 



or, from May to September, with both ; the older acorns are then 

 seen on the older, leafless part of the branchlet, and the young, 

 incipient ones on the younger, leafy part. In oaks with per- 

 sistent leaves some difficulty may arise from the peculiarity 

 that the branchlets which had flowered the previous year, and 

 are now maturing the fruit, often in the second year do not 

 elongate or make new leaves or new wood — in short, do not per- 

 form any function but the maturation of the fruit. In this case 

 the fruit is found near the end of the branchlet, absolutely as if it 

 were an annual fruit ; but the appearance of the leaves as well 

 as of the epidermis of the branch proves them to be over a year 

 old, and wherever a new shoot of the present year can be discov- 

 ered, the difference between this and those of the last year easily 

 solves any doubts. In Q. chrysolepis this peculiarity is quite 

 striking; very rarely (at least in the herbarium specimens exam- 

 ined by me) the fruit-bearing branchlets elongate and again bear 

 flowers, which is the rule in our deciduous biennial oaks. 



The cup of the acorn, an involucral organ, is in all our species 

 covered with imbricated scales, appendicular organs which simu- 

 late bud-scales, and even occasionally seem to assume a pseudo- 

 phyllotactic arrangement. In the Black-oaks these scales are 

 membranaceous and never thickened at base; in the White-oaks, 

 on the contrary, they sometimes have herbaceous tips and, at 

 least the outer and lower ones, are always more or less thick- 

 ened, inflated, or knobby at base ; they are very thick, e.g. in ^. 

 alba and lobata, and very slightly thickened in Q. stellata and 

 Garryana ; in Q. macrocarpa they are herbaceously tipped. 



The shell of the nut or acorn is thinner in the White-oaks and 

 thicker in the Black-oaks ; a much more important and striking 

 character is, that in the former its inside is dark, smooth, and even 

 shining, or rarely pubescent, and in the latter densely silky-tomen- 

 tose, a difference which, I believe, is constant. 



Only one of the 6 ovules of the oak-ovary is developed, while 

 the 5 others persist as small but distinctly recognizable oval, 

 dark colored, pendulous bodies, outside of the seed-coat, in the 

 White-oaks at the base of the perfect seed, in the Black-oaks 

 just below its tip. Only in one of our species, ^>. chrysolepis, 

 are they intermediate or lateral, in some acorns almost basal, and 

 in others scattered over the side from near the base to two- 



