INTRODUCTION 



for example) the primary root dies away annually, 

 and is renewed. The aerial roots of Ivy, and of 

 many tropical plants, and the sucking-roots of the 

 Dodder, are adventitious roots. 



The tubers of the Potato and the bulbs of Onions 



intermediate space is called an internode. The 

 nodes are marked by a thickening of the stem in 

 Knot-grass, and in grasses generally ; and in such 

 plants the nodes are often called knots, and the 

 internodes joints. In grasses, the joints are of 



are not true roots, but underground portions of the 

 stem. 



Stem 



The stalk, stem, or trunk of a plant rises up- 

 ward ; it bears the leaves and flowers, and is 

 generally more or less branched. The points of 

 attachment of the leaves are called nodes, and the 



considerable length ; but in some plants the nodes 

 are so close together that the leaves are crowded 

 into a mass called a " head," as in Lettuce, etc. 



The stem may be either simple or branching, 

 woody or herbaceous, and differs much in shape, 

 etc., in different plants (fig. i, a — i). Thus, the stem 

 may be either cylindrical or angulated, rounded or 

 filiform, erect or decumbent, and so on. 



