288 THE STORY OF PLANT LIFE 



abundant in tropical regions, but are also found in 

 Temperate countries, those found in the latter being, 

 as a rule, terrestrial. The tropical types are largely 

 epiphytic, growing on the stems and branches of trees. 



All are perennial in duration. They include land 

 plants, epiphytes, saprophytes. The mode of branch- 

 ing serves as a means of classification. Some are 

 monopodial, where the main axis continues grow- 

 ing, with flowers on the lateral branches ; others are 

 sympodial, with terminal inflorescence (acranthous) ; 

 others are also sympodial with the inflorescence on 

 lateral axes (pleuranthous). The terrestrial orchids 

 are sympodial. 



In the case of the saprophytic orchids there are no 

 green leaves, as in the Bird's Nest Orchid. They have 

 a fleshy, much-branched rhizome, with or without 

 roots, used in absorption. Many possess mycorhiza. 



Some Orchids are adapted to dry conditions with 

 a reserve store in the form of thickened internodes, 

 or with the bud for the next year developed at the 

 base of the stem, which forms fleshy adventitious 

 roots or tubers which continue during the winter. 



In the epiphytic Orchids, which have to resist 

 drought, there are clinging roots which do not respond 

 to the influence of gravitation, but are negatively 

 heHotropic. There are also absorbing roots, and 

 long true aerial roots. The external cells, epidermis, 

 etc., absorb water. The internal cells are used in 

 assimilation. In drought they hibernate, as pseudo- 

 bulbs, or stem internodes form annually, with the 



