514 



ECOLOGY 



tive, since the plants also possess nutritive soil roots ; these two kinds of 

 roots exhibit the structural contrasts of nutritive and anchoring soil roots, 

 but the differences are more pronounced (figs. 737, 738). In the Eng- 

 lish ivy the anchor roots resemble ordinary adventitious roots in a state 

 of arrested development, and they become transformed into such roots 

 upon coming into contact with moist earth. 



Most epiphytes are anchored to the substratum by roots that may or may not be 

 absorptive. For example, many species of Tillandsia are anchored to branches by 



Fig. 739. — The basal por- 

 tion of a maize stalk (Zea 

 Mays), showing prop roots 

 arising from the lower nodes; 

 note that the stalk becomes 

 thicker upwards. 



Fig. 740. — A strangling fig {Ficus) that 

 began life as an epiphyte, but which now 

 has absorptive ground roots, as well as roots 

 that envelop the tree (Bischqfia trifoliata) on 

 which the fig started ; L^mao Forest Reserve, 

 Philippine Islands. — FromWHiTFORD (Cour- 

 tesy of the Philippine Bureau of Forestry). 



wiry roots that absorb little or nothing from the bark, the leaves being the chief 

 absorptive organs (p. 615). Various ferns thrive equally well in the ground or on 

 trees, but in the latter case they are anchored by ordinary absorptive roots. 



Prop roots. — In various monocotyls, such as the screw pine {Pan- 

 danus), Indian corn, and certain palms, roots issue from the stem at 

 various levels, and grow obliquely downward into the soil; in the air 



