PLANT WORLD CONTINUED 



243 



the plant is young, but they gradually have their margins curve upwards 

 and close. During the time the carpel is closing, an ovule grows out 

 from the base and becomes enclosed by the carpel walls. 



There are several rows of stamens encircling the pistils. Each sta- 

 men or microsporophyl bears four elongated, parallel, sporangia con- 

 taining pollen or microspores. The stalk of the stamen is called the fila- 

 ament, while the four pollen-sacs (sporangia) are known collectively as 



Fig. 147. 



A. Different kinds of pollen grains, highly magnified, two 

 of them forming pollen tubes. ( After Duggar ) . 



B, C. Parts of a stamen. 



A, front ; B, back ; a, anther ; c, connective ; /, filament. 

 (After Strasburger ) . 



D, E, F, Modes of discharging pollen. 



A, by longitudinal slits in the anther cells (amaryllis) ; B, 

 by uplifting valves (barberry) ; C, by a pore at the top of each 

 anther lobe (nightshade). (After Baillon). 



the anther. When mature, the sporangia split longitudinally and permit 

 the escape of the pollen. 



There are two series of leaf-like structures below these we have just 

 been discussing. These two series together form the perianth. The 

 upper series is made up of yellow petals. The petals collectively form 

 the corolla. The lower series consists of five pointed, green sepals, and 

 collectively forms the calyx. 



A spore-mother-cell or archespore arises in the ovule (Fig. 148A). 

 This then divides into four young megaspores, only the deeper one de- 

 veloping. The other three perish. There is thus only a single megaspore 

 in the ovule. The nucleus of the megaspore later divides into two, each 



