CRUCIFER^, 327 



BRASSICA 



Generic Description.— This genus includes annual (black 

 mustard), biennial (turnip), or perennial (cabbages under 

 their natural conditions) herbs. The root system may be 

 fleshy (turnip), or rather woody and solid (cabbages). The 

 basal (radical) leaves are frequently pinnatifid, while those 

 of the stem (cauline) are entire, dentate, or broadly lobed. 

 The large, yellow flowers are in elongated racemes. The 

 sepals, petals, and stamens are as described for the family. 

 The silique (Fig. 133) is elongated, sessile, terete or four- 

 sided, and tipped with an indehiscent, conic, usually one- 

 seeded beak; the valves are convex, one- to three-nerved, 

 the lateral ones often flexuous; the septum (partition) is 

 membranous or spongy; at the tip of the silique is a short or 

 elongated style tipped by a truncate or two-lobed stigma. 

 The seeds are in one row in each cell. 



Pollination. — Representatives of the genus are for the most 

 part insect pollinated. It appears that both self- and cross- 

 polUnation takes place. 



Seedling. — At germination of the seed, the cotyledons are 

 brought above ground. In all representatives of the genus, 

 the cotyledons are emarginate (notched at apex), unequal 

 in size, and three-nerved at the base. 



GeographicaL — There are about 80 species in the genus Brassica, chiefly 

 occurring about the Mediterranean region; some are now cultivated, however, 

 in boreal and subtropical regions of Europe, Asia, Africa, and North and South 

 America. None of the Brassicas are native of America or Australia. 



Key to Principal Species of Genus Brassica 



Leaves of flowering stem not clasping; annuals; sepals spreading. 



Seeds small, reddish-brown; valves of silique one-nerved, B. nigra (black 



mustard). 

 Seeds large, pale yellow; valves of silique three-nerved, B. alba (white 

 mustard). 



