DEPENDENT PLANTS 45 
Do the alga cells seem to be growing vigorously? This 
might indicate whether in the combination the fungus is 
obtaining the greater benefit. 
Root-tubercles.— Examine the smaller roots of the plants 
provided, looking for the bulbous outgrowths upon ‘them. 
In these there are large quantities of bacteria which assist 
the plant by obtaining certain foods which the plant needs 
but can not obtain directly. With good magnification these 
bacteria may be seen. The plant builds around them the 
wall of the tubercle, and from the plant they doubtless 
obtain nourishment and some protection. The plant profits 
greatly by their presence. The ability of clovers and like 
plants to enrich soils depends upon the action of the root- 
tubercle bacteria. 
The field of biology furnishes numerous cases of mutual- 
ism, which will prove of great interest. Additional exam- 
ples may be found in the literature of the subject, and 
should be noted by the student. 
1In order, to be mutualists each form must be in better condition 
than when growing alone. It is not quite clear that this is the case 
in the lichen, for it is generally believed that the fungus alone is bene- 
' fited, holding the alga in slavery. 
