XCII HENRIK HESSELMAN. 
the increased access of light; but we can easily convince ourselves that the 
causes lie deeper. The plants that appear on regeneration spaces, whether 
these consist of small gaps or complete clearings, often consist of such plants 
as gather saltpetre in their tissue, while the same plants do not occur in dense 
woods, or, if they do appear there, are entirely destitute of saltpetre. This 
can be observed both in herbulent spruce-woods, where the nitrogen is normally 
transformed into nitric-acid, and in certain other forests where no saltpetre is 
formed in the humus covering so long as the wood is dense (see HESSEL- 
MAN, 190917). 
These observations thus indicate that a change in the supply of light 
can influence the transformation of the nitrogen in tNetso EA 
detailed study of the conditions under which this takes place is the ob- 
ject of the present treatise. Good guidance in these studies is provided 
by what are called the nitratophilous plants, of which I have given some ac- 
count in a preceding treatise—as, for instance, Rubus Idaeus, Epilobium angusti- 
folium, Galeopsis sp. Arenaria trinervia, etc. By studying the occurrence of 
these plants and investigating their content of saltpetre we have an easily 
available means of estimating the formation of saltpetre in the soil. The re- 
sults thus obtained, however, should be further supplemented by bacteriological 
investigations and direct determinations of the capacity of the soil to form 
saltpetre. 
CHar. III.  Clearings in herbulent coniferous Forests. 
(Detailed description on page 1031.) 
In herbaceous fir-woods of pine as well as spruce saltpetre is formed in 
the soil. As far as can be judged both from the so-called stored samples 
(see HESSELMAN, 1917), and by the nitrate contents of the plants themselves, 
the formation of saltpetre is not so active as in many other plant-associations. 
Despite the shade, which delays the assimilation of the saltpetre absorbed, 
one cannot, as a rule, prove the existence of any nitric acid in the plants 
which form the carpet of the ground. 
In the spruce forests of Jämtland and Lappland, which are developed on 
a Silurian strata and are rich in herbs, there occurs a rather rich ground flora. 
Despite the fact that on storing samples of soil can form not inconsiderable 
quantities of nitrate, it is only quite exceptionally that we can demonstrate the 
existence of saltpetre in the plants. Even the more markedly nitratophilous 
kinds, for example raspberry, often give negative results (HESSELMAN, 1917). 
In these spruce forests an extremely prolific ground vegetation is developed 
on large gaps or clearings. The vegetation easily attains such luxuriance 
as to render very difficult the regrowth of the trees. Fig. 3 gives a very 
good idea of the appearance of such a clearing. The vegetation de- 
veloped is a markedly nitratophilous flora and saltpetre in considerable quantities 
can be demonstrated in a very large number of species. A more detailed de- 
scription of such a clearing is given on page 1031. In the herbulent fir- 
woods, therefore, the increased access of light, whether it is caused by a 
large gap or by a complete clearing, promotes an increased formation of salt- 
petre in the soil. 
