THE ANGIOSPERMAE 1281 



Whatever its cause, and among Angiosperms it is not always due to the 

 operation of the same factors, it is a phenomenon of great biological import- 

 ance and its recognition underlies all consideration of pollination. We shall 

 deal with it more particularly in the Chapter on Sex in Volume III. 



The relative fertility of species has been too little studied in relation to 

 their reproductive power. It is obvious that successful pollination is only 

 one of the conditions involved. Observation shows that flowers in different 

 parts of an inflorescence may vary greatly in fertility, some being invariably 

 sterile, and the same is true of whole individual plants. There may also be 

 long-term causes at work to reduce the fertility of entire species. Perttula 

 has shown that there is a negative correlation between the power of vege- 

 tative propagation and the power of producing fertile seed, and this may be 

 connected with the types of plant association in which a species lives, plants 

 of the more advanced and highly integrated associations having, in general, 

 a lower seed-fertility, and greater activity in vegetative reproduction (see 

 also p. 1351). 



MODES OF POLLINATION 



We have already spoken (p. 1253) of the necessity of an external agent 

 to ensure pollen-transport in cross-pollination, to wit either wind or water or 

 animals, especially insects. We must now consider in more detail those 

 relationships between the floral structure and the pollinating agent, which 

 are generally referred to as " pollination mechanisms ". For this purpose 

 we shall use the following outline classification of the chief types of pollina- 

 tion mechanism, which is modified from that of Knuth. The named types 

 have been selected as far as possible from among the less common examples. 

 Further examples will be described in the following chapters dealing with 

 the Families of the Angiospermae and as far as possible repetition has been 

 avoided. 



Types of Pollination Mechanisms 



I. Anemophily (Wind Pollination) 



{a) Flowers with no stigma (Gymno- 



spermae) Finns, Taxiis, Cycas 



(b) Flowers with stigma 



1. Catkin type Quercus petraea 



2. Pendant type Rnmex crispus 



3. Long filament type Plant ago major 



4. Explosive type Urtica dioica 



5. Motionless type Sparganium ramosiim 



II. Hydrophily (Water Pollination) 



(a) Pollination below water level Ceratophylliun deniersum 



(b) Pollination at water surface Ruppia tnaritima 



