54 THE PEANUT— THE UNPREDICTABLE LEGUME 



tionally indehiscent legume. Under pressure the pod tends to split along 

 a longitudinal ventral suture. Examining cross sections, Richter (59) ob- 

 served that the mechanical tissue of the mesocarp was interrupted along 

 this line. He demonstrated that this was the line of normal dehiscence by 

 cutting the pod into rings and then passing the rings over suitably sized 

 chick peas. The peas were allowed to swell ; the rings were always broken 

 along the suture. 



The thickness of the pericarp or shell and the ease with which it may 

 be broken open, differ greatly in the different varieties of A. hypogaea. 

 The shell may be paper thin or more than 2 mm. thick. There appears 

 to be a positive correlation between size of fruit and thickness of shell but 

 in segregating progenies of thick-large x thin-small peanuts, thick-small 

 and thin-large types occasionally appear. In no case, however, have the 

 writers observed thinnest-largest or thickest-smallest combinations. 



The peanut pod varies in size from about 1 x 0.5 to 8 x 2 cm. and 

 may contain from 1 to 6 seeds. The seeds are suspended from the inner, 

 ventral (upper) surface of the pericarp. The attachment and hence the 

 hilum always lies toward the apex of the seed-bearing segment. A limited 

 elongation may take place in the isthmus between two seed-bearing seg- 

 ments of the pod in some varieties of A. hypogaea. 



Embryo and Seed 



The internal processes leading to the formation of the embryo sac and 

 the embryo were first treated by Reed (55) and with somewhat greater 

 accuracy by Banerji (4) . Embryo sac development and early embryo and 

 endosperm growth have been more recently and fully treated by Smith 

 (68). The frequent occurrence of seed failure, commonly observed as 

 "pops" and one-seeded pods in cultivated peanuts, led Smith to investi- 

 gate the basic reproductive processes which occur in the ovule just before 

 and after flowering. 



In the peanut, a single megaspore mother cell in each ovule under- 

 goes a normal meiosis and produces four megaspores. The lowest 

 (chalazal) spore develops into the 7-celled, 8-nucleate embryo sac, while 

 the other spores degenerate. This embryo sac corresponds to the "nor- 

 mal type" found in other legumes and found in the majority of the in- 

 vestigated cases in flowering plants. By the time of pollination, the 

 synergids and antipodal cells have usually degenerated so that the sac con- 

 sists of only the egg cell and a large central cell containing the two polar 

 nuclei. 



The union of egg and sperm occurs 12 to 16 hours after pollination. 



