244 PLANT LIFE 



plant does the same, but the subsidiary 

 events that have happened during its evolu- 

 tion obscure its record. However, we find 

 two kinds of sporangia, one commonly 

 grouped in a cluster of two or four, and form- 

 ing the so-called pollen sacs borne on each 

 of the stamens. The pollen sacs produce the 

 spores or pollen grains in much the same 

 way as the spores are formed in a fern. But 

 the other sort of sporangium is less easily 

 recognised (Fig. 28). It is often known as an 

 ovule, and the ovules are situated inside a 

 closed cavity called the ovary, which forms 

 the lower part of the pistil of the flower. 

 Each ovule (or sporangium) usually contains 

 but one spore, and this is not, like the pollen 

 grain, thrown out of the sporangium, but 

 germinates inside it, and produces an egg as 

 well as a number of other cells. Moreover, 

 the sporangium is retained within the ovary, 

 and hence the methods of fertilisation which 

 are appropriate for a fern would clearly be 

 impossible here. As a matter of fact, the 

 pollen grain, i. e. the spore from which the 

 male gamete will be derived, has to be brought 

 into special relation with that part of the 

 flower in which the ovule is situated. At the 

 summit of the ovary there is a specialised 

 structure called the stigma. This is often 

 viscid just when the ovules are mature, and 

 thus any pollen which falls on to the stigma 

 is retained. 



The pollen grain or spore already contains 



