SECONDARY ROOTS. 87 



or the Orchis (Fig. 115), where, while one or more of such reser- 

 voirs is exhausted each year, others are providently formed for 

 the next year's sustenance ; and so on from year to year ; a por- 

 tion annually perishing, but the individual plant surviving indefi- 

 nitely. More commonly, the whole body and main branches of 

 the root are somewhat thickened ; or portions of the stem may 

 subserve this purpose, as in all tuberous herbs ; or the nourishing 

 matter may be widely distributed through the trunk, as in shrubs 

 and tree$. These are some of the modifications in this respect of 

 perennial plants, which survive, or at least their roots, and blossom 

 from year to year indefinitely. 



130. Secondary Roots, (Also called Adventitious Roots.) Thus 

 far, the primitive root, that which originated from the base of the 

 embryo in germination, with its ramifications, has alone been con- 

 sidered. But roots habitually spring from any part of a growing 

 stem that lies on the ground, or is buried beneath its surface, so as 

 to provide the moisture and darkness they require ; for such roots 

 obey the ordinary tendency of the organ, avoiding the light, and 

 seeking to bury themselves in the soil. Most creeping plants pro- 

 duce them at every joint ; and most branches, when bent to the 

 ground and covered with earth, will strike root. So, often, will 

 separate pieces of young stems, if due care be taken ; as when 

 plants are propagated by cuttings. Stems commonly do not strike 

 root, except when in contact with the ground. To this, however, 

 there are various exceptions ; as in the case of 



131. Aerial Roots, Some woody vines climb by such rootlets ; 

 as the Ivy, our own Poison Ivy (Rhus Toxicodendron), and the 

 Bignonia or Trumpet-Creeper, which in this way reach the sum- 

 mit of high trees. Such plants derive their nourishment from their 

 ordinary roots imbedded in the soil ; their copious aerial rootlets 

 merely serving for. mechanical support. Other plants produce 

 true aerial roots, which, emitted from the stem in the open air, 

 descend to the ground and establish themselves in the soil. This 

 may be observed, on a small scale, in the stems of Indian Corn, 

 where the lower joints often produce roots which grow to the 

 length of several inches before they reach the soil. More striking 

 cases of the kind abound in those tropical regions where the sultry 

 air, saturated with moisture for a large part of the year, favors the 

 utmost luxuriance of vegetation. The Pandanus or Screw-Pine (a 

 Palm-like tree, often cultivated in our conservatories) affords a 



