362 



PLANT STUDIES 



Fig. 323. Cross - section of 

 anther of a lily {Butomus), 

 showing the separating walls 

 between the members of each 

 pair of sporangia broken 

 down at z, forming a con- 

 tinuous cavity (pollen-sac) 

 which opens by a longitudi- 

 nal slit.— After Sachs. 



The opening of tlie pollen-sac to discharge its pollen- 

 grains (microspores) is called dehiscence^ which means " a 

 splitting open," and the methods of 

 dehiscence are various (Fig. 322). 

 By far the most common method 

 is for the wall of each sac to split 

 lengthwise (Fig. 323), which is 

 called longitudinal dehiscence; an- 

 other is for each sac to open by a 

 terminal pore (Fig. 322), in which 

 case it may be prolonged above into 

 a tube. 



241. Megasporophylls. — These 

 are the so-called " carpels " of Seed- 

 plants, and in Angiosperms they 

 are organized in various ways, but 

 always so as to inclose the mega- 

 sporangia (ovules). In the simplest 

 cases each carpel is independent (Fig. 324, A)^ and is dif- 

 ferentiated into three regions: (1) a hollow bulbous base, 

 which contains the 



ovules and is the (I w\ 



realseedcase, r» W vn 



known as the 

 ovary ; (2) sur- 

 mounting this is a 

 slender more or less 

 elongated jorocess, 

 the style; and (3) 

 usually at or near 

 the apex of the style 

 a special receptive 

 surface for the pol- 

 len, the stigma. 



In other cases 

 several carpels to- 



B 



Fig. 324. Types of pistils 



A, three simple pistils 



(apocarpous), each showing ovary and style tipped 

 with stigma ; B, a compound pistil (syncarpous), 

 showing ovary (/), separate styles (g), and stigmas 

 («) ; C, a compound pistil (syncarpous), showing 

 ovary (/), single style (g), and stigma (n).— After 

 Berg and Schmidt. 



