40 COMPOUND ORGANS OP PLANTS. 



act as props to the stem, giving the tree the appearance of 

 having been raised from the ground. If, however, the tree is 

 under shelter, or cultivated in a stove or hot-house, those 

 thick, strong roots or props, provided by nature, do not 

 develops, but are still seen as protuberances on the surface of 

 the stem. The same phenomena is perceptible on a small 

 scale in the stem of the Zea mays, or Indian corn, the lower 

 joints of which give forth aerial rootlets, which reach or do 

 not reach the soil, according to the amount of support required 

 by the plant. 



In general it may be remarked, that these adventitious roots 

 are developed from those parts of the stem where the nutritive 

 sap encounters some obstacle to its free circulation, and in par- 

 ticular at the nodes or accidental nodosities which exist on the 

 stem or its branches. 



We are able, whenever we please, to produce these adventi- 

 tious roots on the young branches of most ligneous vegetables. 

 It is only necessary to surround the young branch with humid 

 earth contained in any kind of pot or vase. At the end of a 

 definite period, varying according to the species, the roots will 

 develope themselves, and the young branch can be separated 

 and will form another plant. This is a mode of multiplication 

 very useful in horticulture. 



We see the same results produced continually in nature 

 under similar circumstances. Most creeping stems produce 

 roots at every leaf node, that is to say, when there are the 

 suitable conditions, moisture, a certain amount of shade, and 

 immediate contact with the earth ; and the branches of such 

 stems as are vertical, if bent to the ground and covered with 

 earth, almost always take root. This is sometimes done by 

 gardeners, wjio bury the limbs of shrubs by bending down the 



