— 
76 MISSOURI BOTANICAL GARDEN. 
1 mm. on the co-ordinate. Thus, one species has a specific 
gravity between 0.16 and 0.20 inclusive; none occur be- 
tween 0.21 and 0.25; 46 are found between 0.46 and 0.50; 
and 50 lie between 0.71 and 0.75. The majority of species 
(242) occur between the maximal points of 0.46 and 0.75, 
and it is evident that the fall is much more abrupt from 
these common densities to the lowest recorded density than 
to the highest, so that any extension in this direction is 
more remarkable than a corresponding one in the other. 
As the density of wood freed from resin etc., air and 
water, is about one-half greater than that of water, the 
reason for the extreme lightness of the wood of Lettneria 
is to be looked for in connection with its loose structure, 
the softness of its tissues, which are easily compressible 
under the thumb nail, and the absence, at least in the 
largest specimens I have seen, of any heart wood, the tex- 
ture being homogeneous throughout.* It was, therefore, 
subjected to a rather careful microscopical examination, 
with the following results.t 
The pith is nearly round in cross section, although where 
the bundles of the primary wood join it they project 
slightly, giving it a minutely crenulate outline. It is con- 
tinuous and of uniform texture, thus differing from the 
pith of plants like Paulownia and Lonicera which are ex- 
cavated, or Juglans, which is chambered,t and from that of 
Liriodendron, etc., which, though solid, is traversed by 
firmer transverse plates. The cells, as is usual in pith, are 
approximately isodiametric and polygonal, as seen in cross 
section, a little smaller near the periphery ; and in longi- 
* On this general subject see Nérdlinger, Gewerblichen Eigenschaften 
der Holzer, Stuttgart, 1890, 17; and Techn. Eigenschaften der Hélzer, 
Stuttgart, 1860, 115. 
¢ The substance of this paper was presented orally before the 
Academy of Science of St. Louis on May first, 1893, and illustrated by 
specimens of the plant and photomicrographs, some of which are here 
reproduced in half tone or are made the basis of line engravings, show- 
ing the structure of the wood and bark. 
t The ‘‘ discoid pith ’’ of Morren, in Ann. of Nat, Hist. 1839, iv. 73. 
12 
