THE ROOT. 45 



than either of tlie otlier varieties to tlie exofjenoiis steins. 

 The herbaceous species ot' llie Siiiihix aiul the Asparagus af- 

 ford examples of branching ei>nical Endogenous sttnis, and 

 from tlie delicateness of the tibres and the al)undance of the 

 cellular tissue, they would hardly be recognized as belonging 

 to this class of steins. 



52. Acrorrens are such plants as increase by the elongation 

 of their axis without increasing in diameter. The Ferns pre- 

 sent a type of this class of stems. While the Exogens in- 

 crease by external layers, and Endogcns by internal deposi- 

 tion, these seem to increase, as the term Acrogens indicates, 

 by additions to their summit, that is, by tlie simple elonga- 

 tion of their parts. By examining the stem of a Fern we 

 find it composed of cellular substance, and vessels generally 

 bearing in some species a very close resemblance to a vari- 

 ety of spiral vessels, but more observations are necessary to 

 determine the true constitution of this class of plants. There 

 is another variety of formation of Acrogens sometimes call- 

 ed the centrifugal formation, as exhibited by fungi and lich- 

 ens, in which the formation proceeds from a center, the sub- 

 stance being generated nearly upon the same plan. Lichens 

 may often be seen with their centers dead, while the circum- 

 ference is alive and growing. Fairy rings are the result of 

 this formation. 



Section IV. — Root. 



53. The root is that part of the axis of the plant, which 

 descends in its elongation, and is the organ through which 

 the plant receives most of its nourishment, and by which it 

 is attached to the place of its growth. The root in its gen- 

 eral appearance resembles the stem ; and when taken togeth- 

 er they have been, not unaptly, compared to two cones united 

 by their bases. Both taking their origin from the same vi- 

 tal points, and under the influence of the vital power they 

 seem to be endowed with opposite propensities, one growing 

 upwards, seeking light and air, the other with an equal im- 

 pulse forcing its way downwards and burying itself in the 

 earth. We may consider the plant as endowed with opposite 

 polarities. One pole uniformly takinjr the direction of grav- 

 ity, the other as uniformly the opposite direction. The sur- 

 face of the earth may, in general, be considered the equator 

 of this living magnet, and the zenith and nadir its poles. 



Although we speak of Koot and Stem beinnr joined at a 

 point called the neck, still there is no line of demarcalion 



