THE MICROSPORAXGIUM 39 



burger 31 and by Frye. 33 The case of certain orchids, such as 

 Orchis maculata, Calanthe ve rat ri folia, and Neottia ovata, in- 

 vestigated by Guignard, 10 and their allied forms, deserve special 

 mention. Each primary sporogenous cell gives rise to a well- 

 defined mass of mother-cells known as a massula (Fig. 13, A, 

 D), and separated from its fellows by thicker walls. 



The mother-cells and their nuclei usually increase very 

 much in size, and differ from the adjacent cells in their reaction 

 to stains. This growth is usually accompanied by a rounding 

 of the cells and their separation from one another, and also by 

 a thickening of the wall; but in many Dicotyledons (Tropaeo- 

 lum, Althaea, etc.) the mother-cells do not become isolated, and 

 remain packed closely together in the sporangium, due probably 

 to the tardy disorganization of the tapetum or its failure to 

 disorganize. 



The time relations of the events described to those that form 

 the history of the corresponding megasporangium are exceed- 

 ingly variable, but the case of Astilbe, as described by Webb, 34 

 may be taken as an illustration, especially as it probably rep- 

 resents an average case. The microsporangia develop rapidly, 

 maturing in one or two weeks, and precede the megasporangia 

 .stage for stage. The anthers are rounded and enlarged before 

 the carpellary cavity is closed over; the four microsporangia 

 are well marked when the " placentae " are wholly undiffer- 

 entiated ; the tapetum is organized and the mother-cell stage 

 reached when the integuments have not appeared; during the 

 tetrad stage the integuments appear, while the microspores are 

 '" rounded off " before the functional niegaspore is determined. 

 The most extreme cases are probably certain orchids in which 

 pollination occurs before ovules are formed; and oaks (Con- 

 rad 29 ), in which pollination occurs one spring and the ovules 

 do not develop until the next. 



The case of Lemna, as reported by Caldwell, 23 deserves sepa- 

 rate mention (Fig. 14). In the nascent anther a single hypo- 

 dermal group of cells appears as an archesporium and enters 

 upon the usual history, a wall of several layers and a broad spo- 

 rogenous mass being formed. A plate of sterile cells gradually 

 divides this sporogenous mass into two, each of which continues 

 to divide as the anther increases in size. Each of these two 

 .sporogenous masses is divided by a plate of sterile cells, so that 



