372 ELEMENTARY STUDIES IN BOTANY 



tillage to retain the soil moisture during the hot weather when 

 the tubers are maturing. 



There are hundreds of varieties of potatoes, new ones con- 

 stantly replacing old ones. These new varieties are usually 

 produced under exceptionally favorable conditions, so that 

 in ordinary cultivation they degenerate (" run out "). It 

 is for this reason that many of the most notable varieties of a 

 generation ago have been replaced by new ones. 



Roots 



80. Radish. This is one of the most popular vegetables, 

 because it grows quickly and is ready for use very early in the 

 season. Since it can be grow^n at any season of the year and 

 develops in so short a time, it is one of the best garden plants 

 for use in school gardens, as well as in home gardens. The 

 radish belongs to the mustard family (Cruciferse) , which con- 

 tains many common wild flowers as well as useful plants. 

 Associated with the radish in the same family are such useful 

 plants as the mustards, water cress, horseradish, cabbage, 

 and turnip. The family is recognized by its four petals 

 (purple and whitish in radish), its six stamens (two of them 

 shorter than the other four), and its pod fruit, which in the 

 radish is at first fleshy and then becomes dry and corky (Fig. 

 47). 



The races of radish differ in size, shape, color, and texture, 

 as any one who sees them in market will recognize (Fig. 48). 

 The crop is much more uniform if the seed is sifted to get rid 

 of the small and imperfect seeds and of foreign particles that 

 are apt to occur in any package of seeds. The richer and looser 

 the soil, the better. Spring radishes should be sown as soon 

 as the ground can be worked, for the plants are very hardy, 

 and radishes will be ready for use in three to six weeks. In 

 a week or two later they become pithy, so that repeated sow- 

 ings are necessary for a continuous supply. Summer radishes 

 are of slower growth, and therefore keep longer in condition ; 



