8' TUBEROUS AND AERIAL ROOTS. 



plant, in addition to the stems which it elevates into the air, sends 

 out many more below the surface, much after the manner of the 

 runners of a strawberrj', only that they do not extend beyond ten or 

 twelve inches. After a while, these underground stems stop growing, . 

 but sap continues to flow into them from above, and accumulates at 

 the extremity, where it gradually forms the potatoe ; just as a stream 

 of water, suddenly checked by a fallen bank of earth or other obstacle, 

 accumulates behi^^d the barrier, and forms a pond. The real roots of 

 the potatoe are the brown fibres that hang down from it. (Fig. 3.) 

 Many plants form lumpy underground masses of similar nature, though 

 not exactly in the same way. Along with the potatoe, they are called 

 "Tubers" or "Tuberous Roots." The 7m, the wood-anemone, and 

 several others, have a thick underground horizontal stem, technically 

 called a "rhizome." Another modification is seen in the crocus, 

 which is round like the onion, but solid throughout, and called a 

 " cormus." While stems are occasionally produced below the sur- 

 face of the ground, roots -are sometimes put forth from the aerial parts 

 of the plant. Pretty and familiar examples of this occur in the deli- 

 cate green Lycopodiums, now such favourites for green-houses and 

 fern-cases. 



THE STEM. 



The Stem, with the exceptions named above, is the part which rises 

 into the air, generally dividing into branches, and bearing the leaves, 

 the flowers, and the fruit. Whether weak and diminutive, as in the 

 violet, or an enormous woody pillar, as in the trunk of a tree, it is 

 still called the stem, and the boughs, the branches, and the twigs 

 included under the one general name. The chief points to notice 

 in it are the shape, the direction, and the amount of branch. 

 Usually it is round, but in small plants often four-cornered, or three- 

 cornered, and sometimes deeply furrowed. In the everlasting-pea 

 it is curiously winged; in other plants covered with hair, wool, 

 down, thorns, or prickles, as in the rose and bramble. A few, such 

 as the hemlock and dragon-plant, have it spotted and blotched with 

 red or purple. At short intervals along the stem, there are knots or 

 joints. These are called " nodes," and the lengths of stem lying 

 between them, the "internodes." (Fig. 4, o, b.) The direction taken by 

 the stem is very important to notice. Usually it is erect, sometimes 

 remarkably so, as in the Italian j)oplar, but in small ])lants it often 

 lies prostrate, or nearly so, and is then called "procumbent." When 



