326 BOTANY OF CROP PLANTS 



back against the hypocotyl), accumbent (margins folded 

 against the hypocotyl), or conduplicate (folded upon them- 

 selves lengthwise). 



The seeds of mustards, like those of grasses and composites, 

 are short-lived, as compared with those of the mallow family, 

 potato family and pea family. Longevity of seeds is due to 

 a number of factors, chief of which is impermeability of the 

 seed coats to water and oxygen. Seeds with permeable coats 

 are more sensitive to moisture and temperature changes than 

 are those with impermeable ones. When moisture is absorbed 

 by the seed its rate of respiration is increased, and hence its 

 vitaUty reduced. This may be an important factor in 

 shortening the life of the seed. 



Closely Related Families. — Members of the mustard family may be mis- 

 taken for those of the poppy family {Papaveracem) or caper family {Cappari- 

 dacea), both of which are closely related. The poppies have perfect flowers 

 usually with two early deciduous sepals, while capers are distinguished from 

 mustards by the six approximately equal stamens and by the one-celled 

 capsule. 



Key to Principal Genera 



Pod indehiscent, Raphanus (radish). 

 Pod dehiscent into two valves. 



Pod a silique, at least twice as long as wide. 

 Leaves dissected, Sophia (tansy mustard). 

 Leaves broadly-lobed. 



Silique beaked by a persistent style) ; seeds in one row, Brassica 

 (cabbage, turnip, rutabaga, rape, black mustard and white mustard). 

 Silique beakless; seeds in two rows, Radicula (water cress and horse 

 radish). 

 Pod rarely more than twice as long as broad. 



Silique not flattened, nearly circular in cross-section, Camelina (false 



flax). 

 Silique flattened. 



Silique elliptic or oval, Lepidium (penny cress). 

 Silique triangular-obovate or obcordate. 

 Basal (radical) leaves pinnatifid, Capsella (shepherd's purse). 

 Basal leaves entire or merely toothed, Thlaspi (penny cress). 



