182 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



known to come out from the same pollen grain (Stenar, 1925). 

 Eventually, however, only one of them makes further progress. 

 Sometimes the same pollen tube may divide into one or more 

 branches. Such a condition seems to be frequent in the Amenti- 

 ferae, where the branching tubes give the appearance of a ramifying 

 fungous mycelium (see Finn, 19286). In plants whose pollen grains 

 are united into tetrads or into pollinia, several pollen tubes are 

 produced at the same time (Fig. 107 A, B). 



A B C 



Fig. 107. Acacia baileyana, pollinium germinating on stigma (ct = cuticle; g = 

 generative cell ; In = tube nucleus; in = intine). (After Newman, 1984.) B, Cymbi- 

 dium bicolor, germination of pollen grains united in tetrads. (After Swamy, 1941.) 

 C, Elatine triandra, t.s. anther of cleistogamous flower, showing pollen grains ger- 

 minating in situ. (After Frisendahl, 1927.) 



The stigma is believed to play an important part in the germina- 

 tion of pollen, but in many plants germination can also be induced 

 in a sugar solution of appropriate strength. Martin (1913) germi- 

 nated the pollen of Trifolium pratense on hog's bladder moistened 

 with distilled water and suggested that the only use of the stigma 

 lies in controlling the water supply. Katz (1926) agreed with this 

 view and said that the chief function of the stigmatic secretion is to 

 protect the pollen as well as the stigma from desiccation. In her 

 experiments the pollen germinated even on the cut surface of the 

 style, provided the stigmatic secretion was applied to the stump and 

 the latter was kept moist for some time. 



