SOLANUM TUBEROSUM 



545 



layer at first extends across the stem in a transverse direction; 

 but, later, the plane of the layer is curved downward, and the 

 stem becomes swollen below the constriction owing to the multi- 

 plication of the cells in the cortex. 



No definite line of cleavage is formed before the shedding of 

 the flowers, nor is a true cork developed prior to that time. As 

 long as the blossoms appear to develop normally, the cells of the 

 abscission layer remain firmly joined; but, when blasting and 

 yellowing of the buds or blossoms occurs, a cleavage results in 

 the upper portion of the abscission region. The active tissue on 

 the pedicel then forms a callus over the broken surface. Abscission 

 may take place at any stage in the development of the buds or 

 blossoms. 



MicROSPOROGENEsis. — The sporogenous tissue is differentiated 

 early in the development of the anther, and may first be recognized 

 as a mass of small dense cells with large nuclei which occupy 

 each of the four lobes of the anther. The archesporium consists 

 of a layer about two cells in thickness which is parallel to the 

 surface of the anther. It is covered by the epidermis and a variable 

 number of layers of parietal cells. The tissue of the archesporium 

 extends back the entire length of the anther, appearing in cross 

 section as a horseshoe-shaped structure. Since the liberation of 

 the pollen is through terminal pores, there is no modification of the 

 parietal cells to aid in dehiscence. The tapetal cells are large but 

 as the pollen grains mature the entire tapetum and the inner parietal 

 layers become disorganized. The archesporial cells divide several 

 times and the daughter cells are more or less angular in shape. 

 Finally, they lose their angularity and function as microspore 

 mother cells, producing the microspores as a result of two successive 

 divisions, the first of which is heterotypic. 



Rees-Leonard (19) has observed several types of degeneration 

 during the floral development and sporo genesis of the potato. 

 A common type occurs in microsporogenesis in which the sporoge- 

 nous tissue completely disintegrates; and in some of the ovules of 

 such buds, the development of the megaspore mother cell seems 

 to be arrested. Degeneration may occur within the ovule at any 

 stage in development, but is less frequent in very early ones. In 

 some cases, it takes place following the meiotic divisions and all 

 four megaspores disintegrate; while, in others, the degeneration 

 occurs during the development of the megagametophyte. This 



