364 



THE STRUCTURE OF ECONOMIC PLANTS 



die of each sepal lobe is one of the unbranched perianth bundles, 

 and branches from the petal traces form the main laterals. (Fig. 

 184, £.) Additional branching of the mid vein and laterals results 

 in the formation of a number of small veins which extend through- 

 out the undiverged portion of the calyx. (Fig. 184, F, G, H.) 

 Finally, each ultimate calyx lobe is supplied with three principal 

 bundles, each petal and stamen has one main bundle, and the vas- 



sta r 



ri-OV 



Fig. 184. A-H, diagrammatic transections of flower bud from pedicel upward showing 

 arrangement of vascular traces and floral parts: car, carpel; cur tr, carpellary trace; c/x, 

 calyx tube; / sta, inner stamen; ke, keel; o sta, filament of outer stamen; ov, ovule; pet 

 /r, petal trace; /»r;6 /^ perianth trace; /)?^r, provascular ring; .rf /^r, sepal trace ; sta r, stzmiml 

 ring; j/^ /;-, staminal trace ; j^</, standard ; w, vascular strand ; /W, wing. 



cular system of the carpel consists of a large abaxial and two smaller 

 adaxial bundles. (Fig. 183, /.) 



MicROSPOROGENESis. — Sporogenesis in the garden pea has been 

 investigated by Cooper (7), who used two horticultural varieties, 

 Little Marvel and Asgrow's Pride. The microspore mother cells 

 have larger nuclei than the tapetal cells which ultimately become 

 binucleate. The somatic number of chromosomes is fourteen and 

 seven pairs are present at diakinesis. Following the second meiotic 

 division, each microspore mother cell has four nuclei. The division 

 spindles persist for a time and cell plates are formed, after which 

 the walls of the four microspores are deposited. The pollen grains 

 are soon released from the microspore mother cell wall; and, subse- 

 quently, the walls of the former thicken except for two germ pores. 



