172. THE STRUCTURE OF ECONOMIC PLANTS 



In the differentiation of the carpel, the styles develop as two 

 blunt conical outgrowths; and, coincident with this, the upper 

 portion of the carpel grows over and encloses the rounded portion 

 of the floral axis. In so doing, it forms a chamber in which the 

 tip of the axis appears as a protuberance from the inner surface of 

 the ovary; and it is at this point that the single ovule later arises. 



MiCROSPOROGENESis. — The staminal primordia elongate rapidly 

 and there is an early differentiation of a vascular strand in the 

 meristematic tissue. An archesporial cell is formed in each angle 

 of the anther, and this divides periclinally to form the primary 

 parietal and inner sporogenous cells. Radial divisions of the 

 parietal cell produce four cells; and these, with adjoining ones, 

 form a parietal layer which encloses the sporogenous cell. Suc- 

 cessive periclinal divisions of the cells of this layer result in three 

 concentric, parietal layers, the inner one becoming the tapetum, 

 and the outer, the endothecium. At first, the sporogenous cells 

 form a single longitudinal row; but, later, radial and longitudinal 

 divisions occur so that in transection there are about six radially 

 arranged cells which function as pollen-mother cells. As the 

 locules enlarge, the pollen-mother cells become spherical or oval 

 and separate from one another, forming a single layer lining the 

 anther chambers. Finally, meiotic divisions result in the forma- 

 tion of tetrads of microspores. The haploid number has been 

 reported as eight for most varieties of wheat. 



The Development of the Ovule. — The primordium of the 

 ovule first appears as a hemispherical mass arising in a lateral 

 position on the inner adaxial surface of the ovary from a broad 

 placenta. (Fig. 77, 5.) Percival (10) regards it as being derived 

 "from the morphological apex of the floral axis," and states that 

 its lateral position is "due to rapid growth of one side of the axis 

 before the closure of the carpel." As the nucellar tissue increases, 

 two integuments arise at its base, the inner one being formed at 

 about the time that the archesporial cell is clearly difl^erentiated, 

 while the outer one begins to develop before the archesporial cell 

 divides. (Fig. 78, 5.) As growth of the nucellus and the integ- 

 uments proceeds, the more rapid development of the latter results in 

 the complete enclosure of the nucellus. (Fig. 78, C, D.) The 

 outer integument never completely reaches the micropylar opening 

 which is formed by the growth of the inner integument; and the 

 former disintegrates before the maturation of the seed. The 



