164 



MORPHOLOGY OF THE ANGIOSPERMS 



Sequence within the Sporangium. Development of the pollen grains 

 within a sporangium and within an anther sac is commonly simultane- 

 ous, or nearly so, but is successive from one part to the other in some 

 taxa, especially the Ranales and Helobiales. The long anthers of Lirio- 

 dendron show, from one end to the other, successive stages in pollen 

 ontogeny. 



Palynology. The cutinized wall of spores and pollen grains is one of 

 the most resistant of organic structures; it persists as a fossil in peat and 



Fig. 66. Ceratophtjllum submersum L. showing the Triglochin type of pollen-grain 

 development. A, spore; B, C, generative cell formation and beginning of vacuolation 

 before enlargement; D, E, increase in size and vacuolation. (After Wiilff, 1939.) 



in sedimentary rocks of many kinds and all ages — especially in coal 

 and oil shales — when other plant material has been destroyed. Fossil 

 spores and pollen grains are therefore of great importance in the study 

 of the phylogeny of plants. Palynology, the study of spores and pollen 

 grains, has recently been established as a morphological, taxonomic, and 

 phylogenetic section of botanical science and has already contributed 

 information of great value in the determination of relationships among 

 angiosperms. For example, the pollen grains of the Casuarinaceae and of 

 the Amentiferae in general are not of primitive type and support the 

 present generally accepted view that these taxa are advanced, not primi- 

 tive. Palynology is also of much value in studies of historical distribution 

 and ecology. 



