i644 A TEXTBOOK OF THEORETICAL BOTANY 



perianth whorls are probably truly tetramerous. The androecium, on the 

 other hand, may consist of two dimerous whorls, the inner pair having 

 split congenitally into four. This is borne out by the appearance of their 

 rudiments during development. 



It is true that slight variations from the typical flower are found in 

 certain genera. Thus the petals may be reduced or even absent in such 

 genera as Nasturtium, Lepidhim and Coronopus. Two of the outer petals of 

 the marginal flowers of the corymb may be enlarged, giving the flower a 

 zygomorphic form, as in Iberis and Teesdalia. There is an increase in 

 the number of stamens to sixteen in the genus Megacarpaea, while the two 

 lateral stamens are often missing in Cardamine hirsuta. 



The fruit (Fig. 1505) is a capsular pod, which dehisces usually by two 

 valves which separate from below in an upward direction leaving the seeds 

 attached to the framework of the septum, which is formed by the placentas. 



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Fig. 1505. — Types of cruciferous fruits. A, Cheirantlws. B and C, Lumiiia. D, Crawhe. 

 E, Capsella. F, Raphaiius (lomentate siliqua). G, the same in longitudinal section. H, 

 Nasturtium. 



The whole structure is termed the replum. In the majority of species the 

 pod is elongate and is termed a siliqua or, when it is broad and not much 

 longer than it is wide, it may be referred to as a silicula. In the former, 

 the placentas alternate and the seeds lie in a single row in each loculus. 

 In the silicula they are generally formed in two rows. In certain genera the 

 siliqua, before maturity, becomes divided into one-seeded parts by transverse 

 septa, thus in Cakile the siliqua consists of two one-seeded segments, while 

 in Crambe, though two segments are formed, the lower one is barren and 

 forms a continuation of the stalk. Only in Raphamis is a true lomentum 

 of many one-seeded segments normally formed. On the other hand in- 



