STEUCTUKE OF ROOTS. 37 



up a connection between the air and the inner bark, and probably per- 

 form the function of stomata in the advanced period of the growth of 

 the plant. They are considered by DecandoUe and others as being 

 the points where young roots are produced in certain circumstances, 

 and on that account they have been called Rhizogms (g/^a, a root, and 

 ysndiiv, to produce). They are conspicuous in Willows, the young 

 branches of which form roots very readily when placed in moist soil. 

 Some hairs occurring on the styles of plants are called collecting hairs, 

 from the functions which they perform in taking up the pollen. In 

 the species of Campanula, these hairs are so formed that after the 

 pollen has been discharged, their upper part is drawn within the lower. 

 In many hairs, as in the nettle, a circulation of fluids takes place, 

 connected apparently with their nutrition and development (fig. 91). 

 In nettle hairs and in the moniliform purple hairs on the stamens of 

 Tradescantia, or Spiderwort, this movement may be easily seen under 

 the microscope. The subject of the circulation in hairs wiU be con- 

 sidered under Rotation. 



KooT OE Descending Axis. 

 Structure of Roots. 



Before proceeding to the consideration of the special nutritive organs, 

 the root, stem, and leaves, a few remarks are required in reference to 

 the general division of plants into three great classes, Acotyledons, 

 Monocotyledons, and Dicotyledons. The first of these embraces 

 flowerless plants, having a cellular embryo, and no seed-leaf, or, as it 

 is called. Cotyledon. Such plants as Ferns, Mosses, Lichens, Sea-weeds, 

 and Mushrooms, belong to this class. The second includes flowering 

 plants having an embryo with one seed-leaf or Cotyledon, such as Lilies, 

 Palms, Grasses ; while the third includes plants which have two seed- 

 leaves or Cotyledons, such as ordinary forest trees, and the majority 

 of flowering plants. In these classes there are marked difierences in 

 the structure of the nutritive organs, to the consideration of which we 

 now proceed. 



In the young state there is no distinction between stem and root, 

 as regards structure ; both being cellular, and prolongations of each 

 other in opposite directions. In stemless plants, as Thallogens, the 

 root remains in a" cellular state throughout the life of the plants. The 

 root is afterwards distinguished from the stem by the absence of a 

 provision for the development of leaf-buds, and by increasing from above 

 downwards. It is not always easy to distinguish between a stem and 

 a root. Many so-called roots bear at their upper part a portion called 

 their croion, whence leaf-buds arise. Underground stems and roots are 

 often confounded. Some plants, as the Moutan Pseony, the Plum-tree; 



