THE ANGIOSPERMAE 



1363 



There are two reactive end-groups in the molecule and by their means a 

 spatial network is formed by esterification or ether linkages. SporopoUenin 



B 



Fig. 1267.— //)ow;o?.7 purpurea. A, Young pollen grains with general and special walls 

 B, Young pollen grain liberated. C and D, Stages in exine development. E, Matun 



ig pollen grii 

 wall of grain, showing pila of exo-extine. (After Beer.) 



may be identical with the sporonine of Zetsche, which he found in Lycopo- 

 dhim spores and also in the pollen of some Coniferae and in CoryJus. In 

 Piniis sylvestris it makes up nearly 22 per cent, of the weight of the grain, 

 but in Coryliis only 73 per cent. It is of considerable practical interest as 

 the preservation of pollen in peat and in older deposits seems to be chiefly 

 due to membranes containing this substance. 



The intine is variable in thickness, but it is always thicker under the 

 pores, where it protrudes through the pits in the endo-extine and makes 

 contact w^ith the exo-extine. Only the innermost portion of the intine gives 

 the reactions for cellulose; the outer portion gives reactions for pectin. 

 Callose may also be present below the pores. The intine absorbs water 

 readily and swells strongly, especially below the pores where its thickness is 

 greatest. When this occurs it ruptures the exo-extine at the pores and pro- 

 trudes. 



While the layers of the grain wall are developing, the wall of the mother 

 cell and the special walls are breaking down and becoming solvated. Their 

 materials are amalgamated with the materials from the tapetum, which we 



