342 STRUCTURE AND LIFE HISTORIES 



to grow as parasites on other plants (Fig. 2 500) . 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 five stems 

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

 fruit. Npte 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- 



