34° 



THE STRUCTURE OF ECONOMIC PLANTS 



soil. Root tubercles induced by the invasion of a soil organism, 

 Rhizobium leguminosarum, are often present on the root. 



The inflorescence is a few-flowered axillary raceme with an 

 elongated peduncle. In most garden peas, the flowers are white, 

 while in field types, they are purplish, light lilac, or dull white 

 with purple wings and a greenish keel. The calyx tube is oblique, 

 consisting of five sepals which are undiverged at their bases, with 

 four to five somewhat unequal marginal lobes. (Fig. 173.) The 

 papilionaceous corolla is made up of five petals: a large broad 



upper one (the standard) 

 which encloses the others 

 in the bud; two lateral 

 ones (wings) which ex- 

 tend outward obliquely; 

 and two lower petals (the 

 keel) which are more or 

 less united along their 

 ventral edges and surround 

 the stamens and pistil. 

 The diadelphous stamens 

 are ten in number, the un- 

 diverged filaments of nine 

 of them forming an open 

 tube which is cleft on the 

 upper side, while the 

 tenth, uppermost stamen 

 is separate. (Fig. 173, £.) 

 The pistil consists of a 

 single carpel, and the one- 

 celled, superior ovary contains tw^o rows of ovules that arise on 

 adjacent parallel parietal placentae so that they may appear to be 

 in a single row. The slender style is oriented at an approximate 

 right angle with the ovary, and the stigma is bearded on its inner 

 or lower surface. (Fig. 173, C) The plant is entirely self-fertile, 

 and in nature there are probably very few instances of natural 

 cross-pollination. Wellensiek (19) found but one case of spon- 

 taneous crossing in a period of five years, and states that "it 

 appears that the pea is preeminently a self-fecundating plant, 

 which is to be ascribed partly to its being scarcely visited by 

 insects, partly to the early opening of the anthers followed by 



J>^% 



Fig. 171. Stages in development of pea seedling. 



