PLAJSTT REPRODUCTION 



95 



tive nucleus. This usually occurs just before or at the time of 

 the transfer of the pollen grain to tlie stigma. The generative 

 nucleus divides into two sperm or male nuclei. The pollen grain 

 produces a tube which penetrates the stigma and grows down the 

 style. The tube nucleus enters the tube followed by the two 

 sperm nuclei. Their further history will b© followed later (see 

 pages 9Y and 98). 



The position of the ovules with reference to their form and 



Fig. 70. — Types of pollen grain. 



attachments enables us to classify them as follows: {a) The 

 orthotropous or straight ovule, of which the common buckwheat 

 is an example — the ehalaza is at the base and the micropyle at 

 the opposite end. (6) The campylotropous or incurved ovule, of 

 which the common chickweed is an example — the oVule is curved 

 and somewhat crescent-shaped, (c) The ampTiitropous or lialf- 

 inverted ovule, of which the mallow is an example — ^the ovule is 

 not curved, but at right angles to the stem, which extends half 

 the length of the ovule to the ehalaza. (d) The anatropous or 



