ADJUSTMENT TO TEMPERATURE 105 



its associated fungus is termed a rriycorhiza. A similar relation 

 exists between the roots of Leguminosce and the nitrogen bacteria 

 which form tubercles upon them. Symbiosis occurs likewise 

 between many of the simpler algae, such as Pleurococcus and Nostoc, 

 and many cup-fungi and black fungi, constituting the forms termed 

 lichens. In all of these cases the fungus is parasitic upon the 

 host-plant, deriving from it all of its food, or in some cases the 

 larger part of it. Its position in or about the host is of such a 

 nature that it compensates the latter for the loss of food by some 

 beneficial action upon it. The lichen surrounds the algal host 

 with filaments in such fashion as to shield it from heat and drouth, 

 though this relation was first established merely to withdraw 

 carbohydrates from the algal cells. In Leguminosce the bacteria 

 of the root tubercles make it possible for the plant to avail itself 

 of the nitrogen of the air, while without the tubercles, as in the 

 case of all other green plants, this substance can only be obtained 

 from compounds in the soil. In return for the nitrogen fixation, 

 the bacteria take their food supply from the host-plant. In the 

 case of mycorhiza the fungus is probably a saprophyte originally, 

 living upon humus in the soil. After finding its way into the 

 root, it lives more or less parasitically, drawing all of its food, 

 or much of it at least, from the host-plant, and in return aiding 

 the latter in obtaining nitrogen compounds. 



Experiment 34. Nutrition of representative hysterophytes. Prepare 

 cultures by half filling two small Petri dishes with a 2% sugar solution, 

 and two with a thin flour paste. In a third pair place a layer of butter, 

 and in a fourth a piece of moist cooked meat. Spread compressed 

 yeast (Saccharomyces) over the various substrata in one series, and dust 

 spores of blue mold {Penicillium) or black mold (Ascophora) over the 

 material in the second series Place the cultures in a water-bath regu- 

 lated for a constant temperature of 30 C, and note the growth from 

 day to day. Compare the rate and amount of growth on the various 

 substrata. Care must be taken to keep the cultures moist. 



Germinate sunflower seeds, and after the seedlings have appeared 

 sow seeds of dodder (Cuscuta) in the same pot. Note the beha\nor of 

 the dodder seedlings, and study the green thread-like stem in cross- 

 section. After it has become attached, cut a section through parasite 

 and host at the point of union, and study the relation of the tissues of 

 the two. 



Cut longisections of the root-like stems of Corallorrhiza, and note 

 the position and relations of the fungal filaments. Cut cross-sections of 



