X90 THE STRUCTURE OF ECONOMIC PLANTS 



dium of the flower is diverged from the main axis of the inflores- 

 cence as a conical growing point, and shortly thereafter the primor- 

 dia of the sepals are differentiated.^ (Fig- 143? -6.) The growth 

 of the sepal primordia is differential; and the median pair, which 

 become the outer sepals, grow more rapidly than the lateral ones 

 partly enclosing and overarching them. (Fig. 143, D.) The 

 staminal primordia are next differentiated, followed shortly by the 

 carpels, and lastly by the petals. (Fig. 143, E, F.') 



As the carpels develop, a partition is formed which has been 

 termed the dissepiment or septum. In structure this is somewhat 

 variable. At the base of the ovary, it may be thick and solid, 

 while above, it is frequently hollow or double, consisting of two 

 thin walls between which there may be an empty space or some 

 spongy parenchyma. It is thought by some to originate as a 

 secondary placental outgrowth; while others, including Eames 

 and Wilson (x), regard the septum as "an expansion of the ventral 

 margin of the folded solid carpels." 



Fruit and Seed. — The mature fruit is an elongated, conical 

 pod or silique, i to 3 inches in length. It has an extended beak 

 which may equal or exceed the pod itself; and, unlike the majority 

 of cruciferous fruits, is indehiscent. It produces two or three, or 

 sometimes several, seeds which are partially embedded in the 

 spongy tissue of the placentae and septum of the ovary. The ovules 

 are campylotropous, the micropyle being directed toward the 

 stylar end of the ovary. (Fig. 143, H.) The mature embryo is 

 curved and occupies the entire space within the seed coat except for 

 a very small amount of endosperm. The cotyledons are condupli- 

 cate, or folded upon themselves and around the embryonic axis, 

 with the outer, larger cotyledon clasping the inner one. (Fig. 



145) 

 Winton (ii) has described several cruciferous seeds, and Kondo 



(9) has investigated that of the radish in detail. Both agree that 



the seed coat normally consists of four layers, but Winton states 



that "in many species the first and second layers at maturity are 



not distinctly cellular. ' ' Kondo's work included about 30 varieties 



which he divided into three form groups: (i) heart-shaped, (x) 



oval or egg-shaped, and (3) long-oval. (Fig. 144, A, B, C) Type 



* The author is indebted to Mr. Charles H. Quibell, who kindly permitted the use of his 

 materials in the preparation of this section and the accompanying figures on floral develop- 

 ment. 



