CHAPTER XIII 
ECOLOGY 
Ecouoey 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 environment will be briefly discussed. 
SEED DISPERSAL 
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 disarticula- 
ting below the glumes. Among the Andropogonee the 
axis 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 
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