211 
stem must be arrested. When'the new crop of leaves are mature, 
they will begin again to throw down large quantities of cell material, 
and the lining of a new tube will be commenced. During these 
alternations, different sorts of tissues are necessarily formed, and 
the markings on a cross section have the appearance of indistinct 
rings. ; us | ; = 
The Robinia is another notable example of this. I have laid 
upon the table a specimen of an eight year old stem, upon which 
may be counted nine distinct rings and many indistinct ones. It 
was cut in June, and two distinct rings had already been formed, 
one made up of the cells that were forming the inner lining of the 
new tube of wood, and the other made up of the bast cells, forming 
the new layer of bark. It will be found on a close examination of 
the section, that the greater vessels are arranged in concentric layers 
of bundles not in contact, making imperfect or intermediate rings. 
These. bundles are so mingled with the woody tissues as so give the 
layers in some degree a homogeneous appearance, entirely wanting 
in most stems. was 
As has been stated, the accepted theory of Exogenous growth 
is, that a new tube of wood is annually formed outside the last year’s 
growth. This theory is based upon well authenticated observation, 
‘.and no theory in Natural History is admissible, unless it is born out 
of observation and experiment. Ido not presume to offer a new 
one, yet I believe that the old theory will yet undergo great 
modification, as more light is thrown upon this interesting question. 
I do not hesitate to say that from my observations I am forced to 
the conclusion that, as Exogens require.a period of activity and a 
season of rest to form a tube of wood, they may and do form new 
tubes whenever these conditions sharply succeed each other. _ 
0, BR. WILLs, * 
§ 222. Botanical Directory for 1878.—Parts I. and II, contain- 
ing the names and specialties of Botanists, are now ready for dis- 
tribution. Part III., relating to Libraries, Herbaria, etc., will 
probably be ready in May, when it will be forwarded to subscribers, 
- § 223. When the Leaves fall—Mr. N. L. Britton, of New 
‘Dorp, Staten Island, has kindly sent us the following excellent con- 
tribution to this subject. Mr. Britton observes that “the female in 
dioecious plants appears to hold its foliage longer than the male.” 
He has “ noticed this very strongly marked in Ailanthus glandulous, 
Acer saccharinum and A. rubrum, and Salix alba and S. discolor, 
'- but not in Populus. Ilex opaca and Kalmia latifolia hold green 
leaves all winter.” eee w oe seme : 
For want of space we have left out two columns of Mr. Brit- 
ton’s tables; the first, for September 29th, when the leaves were 
all on the plants mentioned except those marked with *, and the 
_ last, for November 30th, when all the leaves were off except those 
marked +. Of the former Cephalanthus had already lost all its 
leaves, September 29th, the others only a few of them; of the 
latter, Myrica cerifera still held most of its leaves, November 30th, 
Quercus palustris had still a. good many leaves, the others had stilla 
few, and Baccharis had still some at Christmas. 
