CHAPTER XIII 

 ECOLOGY 



ECOLOGY is that branch of botany which treats of the 

 relation of plants to their environment. It is often con- 

 sidered to be a branch of physiology since it is a study of 

 the response to stimuli. Plants are acted upon by exter- 

 nal factors, either physical or biological. The response 

 to these forces determines the plant's adaptation to its 

 environment. The more ^important ways in which grasses 

 are influenced by envj^feient will be briefly discussed. 



167. The seeds of grasses are for the most part 

 adapted to dispersal by means of the wind. Some kinds 

 are so small that they are readily transported in this 

 manner without any special adaptation. The fruit by 

 itself (Eragrostis) or inclosed in the lemma and palea 

 (Poa) is easily blown about by air currents. In Panicum 

 and its allies the whole spikelet falls away bY_diaartkiila- 

 ting below the glumes. Among the Andropogonese the 

 alas of the spike usually disarticulates between the pairs 

 of spikelets and the resulting joints are sufficiently small 

 to allow of their being easily transported by the wind. 



168. Dispersal by wind. But the fruit is not infre- 

 quently modified in such manner as to make wind dis- 

 persal more effective. A common adaptation is the devel- 

 opment of silky hairs on some part of the fruit. Such 



(133) 



