114 NUTRITIVE CONJUNCTIVE SY'MBIOSIS 



69. Leaf Tubercles.— Some tropical plants belonging to the family 

 Rubiaceae, especially the genus Pavetta, have small galls, or tubercles, 

 on their leaves that contain nitrogen-fixing bacteria. These bacteria 

 seem to be very closely related to those found in the root tubercles of 

 legume plants and their relation to the host plant is in many ways 

 comparable. The buds of these plants always contain a jelly-like 

 material in which some of the bacteria are present. The bacteria 

 enter the young leaves through stomatal openings while still in the 

 bud. They at once become parasitic and even destroy some of the 

 leaf cells but the formation of a tubercle by the leaf tissue appears 

 to inhibit any further encroachment of the bacteria and many of 

 them are finally digested and absorbed by the leaf cells. 



Since the bacteria are always present on the growing tips of the 

 plant in all of its buds, they are also present in the flowers and there- 

 fore on the seeds. When a seed germinates the bacteria are thus at 

 once present on the growing tip of the plumule. These bacteria 

 differ from the legume bacteria, therefore, in that they spend their 

 entire lives on and in the host plant while the legume bacteria spend 

 a part of their lives as saprophytes in the soil. 



Pavetta plants which were grown for experimental purposes from 

 sterilized seeds and thus were free from bacteria, showed typical 

 "nitrogen hunger." When these plants were inoculated with the 

 bacteria they soon recovered their normal health. This indicates 

 that the symbiotic relation has become practically obligate for both 

 organisms since the seed plants depend upon the bacteria for their 

 nitrogen supply and the bacteria probably cannot live at all without 

 the host plant. 



70. Lichens.— A lichen is a symbiotic combination of fungus and 

 one-celled algse (Fig. 55). It used to be thought that lichens were 

 single unit organisms and they were classified along with the liver- 

 worts. The green cells in them, which are known to be algse, were 

 called gonidia. The fact that the lichen is a plant complex rather 

 than a single organism was discovered by growing the fungi and 

 algffi separately. Later lichens were synthesized by placing lichen 

 fungi among algse that had been growing free in nature. In these 

 cases the mycelium enveloped some of the algal cells and lichens of 

 the usual kind resulted. The algffi that are concerned in lichens com- 

 monly belong to a few well-known genera such as Plcurococcns and 

 Nostoc. The fungi, however, appear to have been greatly modified 

 by the symbiotic relation since they are very dift'erent from any fungi 

 known outside of lichens. 



Although lichens are dual organisms, that is they are made up of 



