6 J: THE CIRCULATION OF NITROGEN IN NATURE. 
we may safely conclude that the rhizobes do possess the 
power of fixing atmospheric nitrogen, possibly when un- 
attached, probably when associated with certain cryptogams, 
certainly when attached to the rootlets of leguminous plants. 
Further, in this interesting instance of symbiosis, the plant 
meets the rhizobe half-way, and constructs a special expan- 
sion of root-tissue, richly supplied with starch, etc., in 
organic connection with its own system, into which at a 
certain stage the nitrogenous compounds manufactured are 
absorbed. It is found too that there are as many species of 
rhizobes as of leguminous plants, or at least that the latter 
succeed best when inoculated with rhizobes derived from the 
same species. 
I have already pointed out that the rhizobes are readily 
cultivated apart from the plant, and this is now done on 
t-he commercial scale, cultures being sold expressly for the 
purpose of enriching the soil, under the exceptionally ill- 
selected name of nitragin. I exhibit three specimens of 
nitragin. 
Our next step is to inquire what becomes of the nitrogen 
thus withdrawn from the air, and bound up in plant tissues. 
The life of a plant is terminated in one of two ways — by a 
natural death or by ingestion by animals. 
In the former case disintegration is a comparatively slow 
process, at any rate as compared with the corresponding 
change in animal tissues. You are, of course, well aware 
that it is quite easy by a comparatively imperfect process of 
desiccation to preserve most plant tissues in a fairly natural 
condition. 
Before the plant dies, if it has run its proper course, much 
of the accumulated nitrogen is stored up in the seed ; that 
portion, however, which remains in the stem, leaves, and roots 
is largely evolved as the element during the slow decay. In 
