ANALOGY or PLANT OBGAXS. 51 



3. F/ii/sioIoffi/ of root li. — (1) The clongntion of roots is confined to 

 a space of abuut one line in length at u distance of half u line from 

 the actual apex ; a growing stem on the other hand appears to 

 elongnte by growing throughout its whole length. (2) New rootlets 

 are always endogenous. Arising from the pericumbium, they burst 

 through the tissue, raising the cortical layer into a " coleorhiza " 

 round the base of the emerging rootlet. Regularly formed buds, on 

 the other hand, are exogenous and form superficial papillae near the 

 pioiction vegetationis of a stem. Adventitious buds, however, are 

 endogenous. (3) The functions of I'oots arc (i) to fix the plant 

 to its site in the earth, or as an epiphyte on other plants ; or on 

 rocks, etc., in water or air ; unless the plant fioat freely on or is sus- 

 pended in water ; (ii) to absorb water with mineral and organic 

 matters in solution: the modifications being («) normal, in soils 

 and water, (h) absorbents of organised matter, as in Neottia Nidus- 

 avis and " saprophytes," (c) aerial absorbents, as in orchids, [d) para- 

 sitic, either subterranean, as in Orohanche, or aerial, as in Cuscuta ; 

 (iii) to act as 2J>'ops for climbing, as on ivy and orchids ; (iv) to 

 form reservoirs of nutriment, as tuberous roots ; (v) to act as 

 propagating instruments, whenever buds are produced on roots, as in 

 Anemone Jnponica ; wdile Ranunculus Fiearia and terrestrial orchids 

 propagate by the production of adventitious tuberous roots, each 

 being terminated above by a bud. (4) The vitality of roots is 

 generally very great as compared with that of stems and any other 

 part of the plant. Lindley * mentions live roots having been found 

 in land many years after the trunks to which they belonged had 

 been destroyed, as of white thorn. Knight found evidence of the 

 same fact in fruit trees. 



Roots agree with stems in forming similar concentric woody layers 

 with cambium and bark, and having the individual cells of the 

 different layers of the same nature as, but larger in many cases than, 

 the cells in the stems : thus the wood-cells of the root of Pinus 

 have two to four rows of disks, whereas they are in a single row on 

 the wood-cells of the stems. Roots can produce buds. Many plants 

 habitually do so, as Anemone Japonica, Pyrus Japonica, peach and 

 plum trees. The roots of Neottia are said to bear leaves, f and when 

 roots of trees, as elm, horse-chestnut, etc., become exposed, they 

 then will produce buds freely. An adventitious root proceeding 

 from the upper part of a decorticated spot on a Rohiuia (false 

 acacia) penetrated the soil at a distance of five feet. Being detached 

 above, it threw out leaf-buds and now forms a small tree, the true 

 root assuming the character and functions of an aerial trunk. ij: 



On the other hand, exogenous stem-structures differ from roots 

 in having, if complete, the following eLments: — medulla or pith, 

 regular medullary rays, wood in regular concentric cylinders, a 

 cambium layer, liber and soft bark, a green layer, liber and 

 epidennis with stomata, and no apical cap. In its method of growth, 



* ' Tlieory of Ilorticulture,' p. 31. 



t Perliaps these are stem-structures. 



X This is growing in the Rectory garden at Ealing. 



