190 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



branches. In Coffea arabica (Mendes, 1941) the pollen tube does 

 not bifurcate but shows two subterminal openings through which 

 the two male gametes are discharged into the cavity of the embryo 

 sac. 



Rate of growth of pollen tube. 1 A considerable amount of in- 

 formation is available on the subject but most of the older records 

 are rather vague and perhaps only rough estimates. It must also 

 be kept in mind that the rate of growth of the pollen tube is affected 

 to an appreciable degree by the environmental conditions prevailing 

 at the time of observation, and most of the existing data are there- 

 fore to be considered as approximate only. 



In certain members of the Fagaceae and Betulaceae several 

 months elapse between the time the pollen grains alight on the 

 stigma and the time the pollen tube enters the embryo sac. In 

 some species of Quercus (Bagda, 1948) this period may be as long 

 as 12 to 14 months. In Hamamelis virginiana (Shoemaker, 1905) 

 pollination occurs in late autumn, and at the beginning of winter 

 the tips of the tubes reach near the base of the funiculus. But here 

 they "hibernate" for the rest of the winter and growth is resumed 

 only in the spring. Fertilization takes place in May, about 5 to 7 

 months after pollination. In Alnus glutinosa and Corylus avellana 

 (Benson, 1894) 3 to 4 months elapse between pollination and fertil- 

 ization. Since such long periods are also found in several gymno- 

 sperms, it is tempting to infer that this is a primitive feature, but 

 long intervals are also known in the Orchidaceae where the ovules 

 are not even formed until after pollination has taken place. To 

 mention a few examples, in Paphiopedilum maudiae (Duncan and 

 Curtis, 19426) approximately 19 to 20 weeks elapse between pollina- 

 tion and fertilization; in P. villosum (Duncan and Curtis, 19426) the 

 period is 14 weeks; in Phalaenopsis pamala (Duncan and Curtis, 

 1942a) and Dendrobium annosum (Pastrana and Santos, 1931) about 

 10 weeks; in Cattleya spp. (Duncan and Curtis, 1943) about 6 weeks; 

 in Cypripedium parviflorum (Carlson, 1940) 26 to 33 days; in C. 

 pubescens (Duncan and Curtis, 19426) about 4 weeks; and in Orchis 

 maculatus (Hagerup, 1944) about 2 weeks. 



Fairly long intervals between pollination and fertilization are 

 also known in plants belonging to other families. In Garry a elliptica 



1 For a more detailed account of this topic, see Finn (1937a). 



