364 



GERMINATION OF POLLEN 



cell sooner or later follows suit (Fig. 215, C). The former usually 

 remains undivided, whilst the latter ultimately produces two 

 male nuclei (cf. below). 



In its downward growth the pollen hibe is sometimes nourished 

 by the secretion of papillae forming a continuous lining to the 

 canal or canals traversing the style and extending uninterruptedly 

 into the loculi of the ovary {e.g. Tulip and Violet). In many 

 cases, however, the style is solid, the pollen tube penetrating 

 through the axial core of thin-walled tissue, nourished by the 

 sugary sap which the latter contains. In some cleistogamic 



Fig. 213. — Diagrams illustrating normal fertilisation (porogamy, A) and 

 chalazogamy (B). e.s., embryo sac ; m., micropyle ; ii., nucellus ; 

 pi., pollen tube. 



flowers {e.g. Purple Deadnettle) the pollen grains germinate in 

 the anthers, and the pollen tubes grow from there to the stigma. 

 The period of time occupied by the pollen tube in growing 

 down to the ovules varies greatly in different species, and bears 

 no relation to the distance to be traversed. In the Crocus, 

 where the style is 6-10 cm. long, the interval is only from one 

 to three days, whilst in the Meadow Saftron {Colchicmn), with 

 a style of about the same length, it is six months (from autumn, 

 when polhnation occurs, to the following spring). A similar 

 prolonged interval not uncommonly obtains in woody plants 

 {e.g. Hazel), though the distance traversed is short, and in some 

 Oaks the ovules do not even commence development until after 



