80 MORPHOLOGY OF ANGIOSPERMS 



the mother-cell very rarely fails to divide, but that there is a 

 strong tendency to suppress one of the divisions and form a 

 row of three megaspores. 



Among the Sympetalae the complete tetrad appears with 

 remarkable uniformity. This is associated with a very small 

 nucellus, most frequently only the epidermal layer investing 

 the tetrad row, and the suggestion is evident that there may be 

 some causal relation between these two facts. Occasionally, 

 however, one of the divisions is suppressed, and a row of 

 three megaspores is the result, the only cases we have found 

 being Vaccinium and Lycium (Vesque 4 ), Lobelia (Marshall- 

 Ward 8 ), Lonicera and Nicotiana (Guignard 12 ), and Trapella 

 and Sarcodes (Oliver 21 ' 24 ). Among the Rubiaceae Lloyd 105 

 reports that while each mother-cell forms a tetrad there are 

 usually no walls (Fig. 33), as in Avena (Cannon 86 ) and Eich- 

 hornia (Smith 53 ). Among the Verbenaceae Treub 14 reports 

 that in Avicennia officinalis the mother-cell does not divide ; in 

 Aphyllon uniflorum Miss Smith 102 reports that' the mother- 

 cell does not divide, although Koch 19 figures a row of four 

 megaspores in Orobanche ; in the parthenogenetic Antennaria 

 alpina Juel 74 finds that the mother-cell does not divide, how- 

 ever, in A. dioica, in which fertilization regularly occurs, a 

 row of four megaspores is formed. Undoubtedly more numer- 

 ous exceptions will be found, but the evidence seems clear that 

 the complete row of four megaspores is almost universally pres- 

 ent among the Sympetalae. 



As has been stated, the reduction in the number of chromo- 

 somes occurs during the first mitosis in the megaspore-mother- 

 cell, whether a row of four, or three, or two megaspores is to 

 be formed, or the mother-cell is to function directly as a mega- 

 spore. In Lilium, the first described form in which the mother- 

 cell does not divide to form megaspores, the beginning of a 

 cell-plate is clearly visible in the spindle during the first mito- 

 sis, and at the second mitosis there is also a rudimentary cell- 

 plate. Since the other cytological characters of these two mito- 

 ses are identical with the first two mitoses in forms that have' 

 the row of four megaspores, it might be suggested that the 

 rudimentary plate is a survival, indicating that the ancestors 

 of Lilium once produced the row of four, and making Lilium 

 in this respect a specialized rather than a primitive form. This 



