226 
that nearly all important works or papers on paleobotany come 
to the library as soon as published. 
Every year students and investigators in paleobotany have 
availed themselves of the opportunities and advantages which the 
Garden supplies in this line of work, which are believed to be 
unequalled by any other institution in America, and which could 
be still further enlarged by including within their scope equip- 
ment for morphological as well as systematic work. 
THUR HOoLticx. 
Curator. 
ENRICHING SOIL BY CRIMSON CLOVER. 
value of clovers and other leguminous plants in the ferti- 
lization of soils has long been recognized but it is only within 
comparatively recent years that the reasons for this have been 
thoroughly studied. 
Nitrogen which is contained in the soil in the form of com- 
pounds is necessary to the life of the plant, and although free 
nitrogen is present in the air in great abundance it is not avail- 
able to the average plant inthis form. When the nitrogen com- 
pounds become exhausted from the soil by constant use it be- 
comes necessary to restore these through fertilizers. Although 
free nitrogen is not available by the average plant it has been 
found that certain bacteria which are known as nitrifying bacteria 
are able to use the free nitrogen from the air and to fix it in the 
form of compounds in which form it is available by other plants. 
These bacteria do not act alone but live as parasites on the roots 
of clovers and related plants where they form swellings known 
as nodules. Although parasites, they give in return for the sus- 
tenance which they draw from the plant on which they grow, the 
nitrogen so necessary to that plant. Through this adaptation 
leguminous plants are able to grow in soils which contain a very 
small amount of nitrogen compounds and to restore to the soil 
through their remains these compounds in sufficient quantity to 
supply the needs of other forms of vegetation. By the growth 
and the subsequent plowing under of leguminous crops it has 
