76 BOTANICAL GAZETTE [JULY 
lines along which future investigation should lie, especially in view of the fact 
that KENDALL succeeded with lower percentages of alcohol in bringing down a - 
different type of precipitate. This latter precipitate might be expected to 
yield cytolytic enzymes. He also finds a reduction in the sugar content of 
abscission zones following cell separation, and that the normal acidity on 
Nicotiana pedicels is low and is only slightly reduced during abscission. This 
latter fact is taken to indicate that the activity of enzymes alone is responsible 
for the dissolution of the middle lamellae during cell separation. 
KENDALL reports that illuminating gas and laboratory air will cause abscis- 
sion in the majority of the species investigated, but that resistance to abscission 
stimulated in this manner appears suddenly in some species. Tests were also 
made as to the effect of a variety of mutilations of the flower and pedicel in 
cing abscission. Relatively slight injuries to the ovary were effective, 
whereas considerable amounts of tissue had to be removed in the case of other 
flower parts before abscission was induced. It is interesting to note that 
mechanical injury was not found to be particularly effective in the tomato, and 
that the following species rarely or never exhibit floral abscission: Nicotiana 
Bigelovii (3 varieties), N. quadrivalvis (2 varieties), N. multivalvis, Petunia 
hybrida, Salpiglossis sinuata, Salpichora rhomboidea, and Lycium australis. A 
detailed summary of the pertinent literature is included in KENDALL’s paper. 
—T. H. GoopsPeep. 
Nitrates in forest soils and forest regeneration.—In an important contri- 
bution HEssELMAN* has reviewed the present state of our knowledge of the 
composition of forest soils and finds, among other things, that while from earth 
containing relatively little humus it has been possible to isolate organic com- 
pounds of known composition the humus of many soils is composed largely of 
chemical compounds of undetermined character, but that on the whole the 
constituents are colloidal in nature and are largely influenced by the amount 
of mineral salts in the soil and ground water. He distinguishes two types 9 
forest humus soils, the ‘‘mild humus” characteristic of deciduous forests, well 
aerated and containing nitrate-forming as well as denitrifying bacteria, an 
“raw humus” found in coniferous forests as a series of layers of leaves and litter 
in various stages of decomposition from which nitrate-forming and denitrifying 
bacteria are usually absent. 
Recognizing decomposing litter as one of the principal sources of nitrogen 
in forest soils, he has investigated the “decay capacity” of various forest types, 
using several different methods. He has determined the relative abundance 
of various bacteria, the ere content of trees and sip and has shown 
that nitrate supply and nitrate formation is at n beech forests and 
at its minimum in mossy eg stands. Lime in the soil and in solution 
os oa Seeds Henrik, Studier ver saltpeterbildningen i naturliga jordmaner 
och dess betydelse i i vaxteekologiskt avseende (with abstract in German). Meddel. 
fran Statens Skogsférséksanst. Haft. 13-14. 297-527. pls. 7. figs. 30. 1917- 
