6i8 THE STRUCTURE OF ECONOMIC PLANTS 



an outer layer of cells, the latter by subsequent divisions forming 

 the tapetum, the middle layer, and the endothecium. Gates and 

 Rees (lo) report that there are from 15 to lo pollen mother cells 

 producing a total of about 60 pollen grains. They observed that 

 the pollen mother cells frequently separate before synapsis and 

 this is confirmed by Jones. The walls of the microspores are 

 formed by furrowing which is initiated at the periphery of the 

 mother cell and extends centripetally until the four furrows meet. 

 Following the secretion of the walls of the microspores, the 

 original wall of the mother cell disintegrates, and at about this 

 time the tapetal cells become binucleate and later quadrinucleate. 

 The nuclei may be more or less completely disintegrated even 

 prior to the disorganization of the tapetal walls and finally a 

 Plasmodium is formed. The microspores continue their develop- 

 ment within the anther and undergo nuclear division so that at 

 anthesis each pollen grain possesses a vegetative nucleus and two 

 slender filamentous microgametes. 



Pollination. — The floral buds elongate very rapidly in the 

 14 hours preceding anthesis, and the involucral bracts enclosing 

 the head begin to open at the summit because of the development 

 of the individual flowers within. The flowers open early in the 

 morning and remain so for about two hours, after which the corolla 

 closes tightly and does not again open. All the flowers of a 

 single head open at one time; and, two or three days later, the 

 corollas, stamens, and the wilted styles and stigmas are shed. In 

 California, where a large proportion of the lettuce for seed purposes 

 is grown, the crop is usually ready for harvest in July or early 

 August; but, in some places, the flowering peak may occur in 

 June and individual plants may produce flowers over a period of 

 two months or more. 



The mechanism of pollination which has been described by 

 Knuth (15), Jones (13), Oliver (19), and Thompson (17) is such 

 that the flowers are almost entirely self-pollinated. Natural 

 cross-pollination has been observed, however; and, on the basis 

 of genetic experiments, Thompson found that natural crossing 

 occurred in zo different populations in percentages ranging from 

 1.33 to 6.1.-L, indicating that "the extent of natural crossing may 

 be greater than is generally believed." Insect visitations have 

 been observed in several instances, which also suggests that there 

 is opportunity for cross-pollination. 



