LACTUCA SATIVA 



617 



m m c 



^'-'in m c 



—int 



-m m c 



Megasporogenesis. — In most cases, a single ovule arises at 

 the base of the ovary, but the occurrence of tw^o or more ovules is 

 not uncommon. There is an early differentiation of a single 

 hypodermal archesporial cell which, according to Jones (13), 

 functions as a megaspore mother cell. (Fig. 3x7, A.') The 

 nucellar tissue consists of 

 a single layer; and it never 

 becomes thicker, since its 

 cells divide only anti- 

 clinally as the megaspore 

 mother cell enlarges. The 

 one massive integument 

 begins to differentiate at 

 about the time the mega- 

 spore mother cell is clearly 

 differentiated, and its uni- 

 lateral growth results in a 

 gradual curvature of the 

 ovule until it becomes 

 completely anatropous. 

 (Fig. 3^7, B, C.) This is 

 accomplished by the time 

 the first or heterotypic division of the megaspore mother nucleus 

 occurs. 



The formation of the linear tetrad of megaspores is similar to 

 that reported for related genera. The three micropylar megaspores 

 disintegrate and the innermost one is functional forming an eight- 

 nucleate megagametophyte by successive nuclear divisions. Jones 

 did not observe an early fusion of the polar nuclei as is reported for 

 some composites, and states that it occurs at the time of fertiliza- 

 tion. Although the number of antipodals has been reported as 

 variable for other members of this family, he was unable to observe 

 more than three in any instance. This seems to be the constant 

 number for most of the investigated Cichorieae. A conspicuous 

 integumentary layer, which persists until the seed is almost 

 mature, surrounds the megagametophyte and is regarded as having 

 a nutritive relation to the developing embryo. 



MicROSPOROGENESis. — Microsporogenesis is initiated by the 

 development of a single row of hypodermal archesporial cells. 

 These divide periclinally to form a row of pollen mother cells and 



QS''"nls 



Fig. 317. A-C, stages in development of ovule: 

 inf, integument \ mm c, megaspore mother cell ; nh, 

 single-layered nucellus. (After Jones, Hilgardia?) 



