562. THE STRUCTURE OF ECONOMIC PLANTS 



agency of insects, the bumblebee, according to Fink (9), being the 

 most common visitant. 



The growth of the pollen tube is relatively slow even at optimum 

 temperatures. According to Smith and Cochran (19), these are 

 70° to 85° F. for the best germination and 70° for the maximum 

 growth of the pollen tube. Under favorable conditions, no case 

 was found by them where fertilization could be observed in less 

 than 50 hours after pollination. Germination and growth were 

 very poor on either side of the optimum temperatures at 50° and 

 100° F. respectively. 



Megasporogenesis. — Many ovules are developed on the massive 

 fleshy cauline placentae, and they completely cover the free surfaces 

 at about the time that the microspore mother cells undergo the 

 first reduction division in the anthers. The ovules develop as 

 erect globular protuberances on the placental surface; but the 

 growth of the ovule is not uniform, and it becomes anatropous as 

 a result of a more rapid cell division on one side of the primordium. 

 (Fig. X93, A.^ It develops a single, massive integument that 

 completely encloses the nucellus, except for the micropylar opening, 

 by the time the four megaspores are formed. The cells divide in 

 all planes so that the integument increases in thickness as well as 

 in length. A single vascular strand is differentiated from the 

 placental bundles which passes through the funiculus terminating 

 in the chalazal region of the ovule. 



According to Cooper (8), the hypodermal archesporial cell does 

 not divide to form a primary parietal and primary sporogenous 

 cell; but functions directly as the megaspore mother cell. By the 

 time young microspores have developed in the anther sacs, the 

 megaspore mother cell is undergoing its first or heterotypic divi- 

 sion; and, as a result of reduction divisions, a linear row of four 

 megaspores is formed. Of these, the megaspore at the chalazal 

 end forms the megagametophyte and the other three disintegrate. 

 The growth of the megagametophyte is rapid, and three suc- 

 cessive nuclear divisions result in the formation of an eight- 

 nucleate gametophyte, after which the two polar nuclei fuse so 

 that the mature megagametophyte is seven-nucleate. The two 

 synergids and the megagamete are located at the micropylar 

 end of the gametophyte, while the three more or less triangular 

 antipodal cells lie at the chalazal end and have a tendency to dis- 

 integrate rather early. As the nucellar tissue is resorbed, the 



