BROOM-RAPES 231 



is best represented in the warm temperate regions of the old 

 world ; about half a dozen species of Orobanche, the broom- 

 rape, occur in Britain. Orobanche rubra is parasitic on 

 roots of the thyme, Orobanche major on leguminous shrubs, 

 Orobanche caryophyllacea on bedstraws and brambles. 

 Some species are thus restricted in the choice of a host, 

 others are less exigent. 



The inflorescence axis appears above the soil ; in 

 Orobanche major it is i to 2 feet high and stoutish, in other 

 species it is smaller. It bears a few small scales and ends 

 in a crowded spike. Below the surface it terminates in a 

 swollen base with many scales. The plants possess no « 

 chlorophyll and have dull colouring in tints of brown, yellow, 

 and purple. Brilliant colours are developed in other 

 members of the family, the Caucasian Phelipaeas having 

 bright scarlet flowers. 



The seeds of the Orobanchacege are minute, with an 

 undifferentiated embryo embedded in an endosperm. 

 Scattered by the wind, they are washed into the soil by rain, 

 and germinate only in contact with living roots of host 

 plants. A chemical effect must be involved, but of what 

 nature we do not know. A filamentous embryo i mm. 

 long is produced, in which only relative position distin- 

 guishes a root end from a shoot end ; the latter remains 

 in the seed coat, the former comes in contact with the host 

 root and sends a sucker down to the wood. Vascular 

 tissue then develops in the seedling, and an extremely 

 intimate connection is made with the host, wood with wood, 

 bast with bast, cortex with cortex, and epiderm with epiderm. 

 The upper part of the seedHng now grows into a tiny tuber 

 with a lumpy surface, and the apical portion usually withers 

 away. From this tuber arise adventitious outgrowths 

 which extend to other host roots and produce secondary 

 suckers. They arise exogenously, and, if a root cap is 

 present, it is much reduced. They may, perhaps, be looked 

 on as adventitious roots of an aberrant nature. The 

 flowering stem arises as a rule singly and adventitiously 

 from the tubers. Flowering stems may also arise from the 



