342 



STRUCTURE AND LIFE HISTORIES 



to grow as parasites on other plants (Fig. 2500). The con- 

 dition to success in such experiments is that the osmotic 

 strength of the cell-sap of the host must be less than, or 

 at least not greater than that of the parasite. 



Fig. 250, — Cross-section of a branch of live oak, showing live stems 

 of mistletoe, parasitic on the oak; the upper stem with foliage and 

 fruit. Note the prominent "sinkers" of the parasite, some of them 

 growing laterally for a short distance, close under the surface of the 

 bark, and then radially, deep into the tissue of the wood. 



313. Fungal Parasites. — Mention has already been 

 made in Chapter XIV of the parasitism of the entire group 

 of fungi, including the smuts, rusts, and other disease- 

 producing fungi, on flowering plants. The ''shelf-fungi," 

 commonly found on forest trees, are economically impor- 



