656 A TEXT-BOOK OF BOTANY. 



Cranberry is the fruit of several species of Vaccinium which 

 are sometimes grouped in a separate genus, Oxycoccos. There 

 are two principal species: The large or American Cranberry (V. 

 macro carp on) in which the berries are ovoid or oblong and the 

 small or European Cranberry ( V. Oxycoccos} in which the berries 

 are globose. The berries contain from 1.4 to 2.8 per cent, of citric 

 acid; and a bitter glucoside, oxycoccin (Fig. 359). 



Many attempts have been made to cultivate the blueberry, 

 trailing arbutus, and other plants of the Ericaceae. For some 

 years a number of the agricultural experiment stations in the 

 United States have attempted to grow the blueberry as a fruit, 

 but none of these attempts has resulted in the commercial success 

 of blueberry culture, and the experimental results have been chiefly 

 of a negative character. The reason for this has been due, as 

 pointed out by Coville (Bull. No. 193, Bureau of Plant Industry, 

 U. S. Department of Agriculture), to a misunderstanding of the 

 soil requirements for this plant. Plants will thrive only in soil 

 having the following properties : I. The soil must have a distinctly 

 acid reaction, such as is found in peat, bogs or on the surface of 

 the ground in sandy, oak, or pine woods. 2. Aeration of the soil 

 is necessary. The rootlets of the swamp blueberry are remarkable 

 in having no root hairs whatsoever, so that their absorptive surface 

 is only about one-tenth that of other plants having root hairs. 

 The growth of the rootlet of the blueberry is much less than that 

 of other plants, being about at the rate of only i mm. per day under 

 favorable conditions. The rootlets of healthy blueberry plants are 

 inhabited* further by a mycorrhizal fungus which apparently has 

 the property of assimilating nitrogen. 



II. ORDER PRIMULALES. 



Of the three families belonging to this order, there are two 

 which are to some extent represented in temperate regions. 



a. PRIMULACE^ OR PRIMROSE FAMILY. The plants 

 are mostly perennial herbs with perfect regular flowers, and capsu- 

 lar fruits. The family is chiefly of horticultural interest, as it 

 contains the genera Primula and Cyclamen. There are several 

 species of Primula cultivated, and they are among the most popular 

 and beautiful of the florist's flowers (Fig. 360). Several of the 



