FERTILIZATION 



185 



protrudes out of the micropyle so that the pollen tube comes in 

 direct contact with it. In several members of the Loranthaceae 

 there is no integument and therefore nothing that can be called a 

 micropyle. Here the embryo sacs undergo a remarkable elongation 

 and meet the pollen tubes at some point in the stylar region (see 

 also p. 143). 



In some plants the pollen tube enters the ovule through the 

 chalaza. This condition, known as chalazogamy (see page 17), 

 was first reported in Casuarina (Treub, 1891) and soon afterwards 



ABC 



Fig. 109. Development of obturator in Acalypha indica. A, l.s. terminal flower 

 of inflorescence, showing ovule at megaspore mother cell stage. Note beginning 

 of formation of obturator (shaded). B, l.s. lateral flower at more advanced stage. 

 C, ovule, enlarged to show hood-like obturator fitting over nucellus. (After 

 Maheshwari and Johri, 1941.) 



in several members of the Amentiferae. Nevertheless, it is not 

 confined to them, being also known in Rhus (Grimm, 1912), Cir- 

 caeaster (Junell, 1931), and a few other genera. Recent studies have, 

 however, shown that even in such cases, where entry into the ovule 

 is effected through the chalaza, the tube usually continues its 

 growth over the surface of the embryo sac and penetrates it only 

 after arriving near the egg apparatus. As examples may be men- 

 tioned Ostrya carpinifolia (Finn, 1936), Juglans regia (Nast, 1941), 

 and Casuarina equisetifolia (Swamy, 1948) (Fig. 108). 



In Alchemilla (Murbeck, 1901), Cucurbita (Longo, 1901; Kirk- 



