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 five 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- 
