ROOT 



part*. Near tin- aprx is the growing-point, a mass 

 of actively dividing cells, called the primary ni--ii 

 Mem ; by ilivisinii these give rim to, below, nevcta! 

 layers of simple celU (the root-cap), aKove, tin- main 

 moss of tin* root, which, immediately II!KIVC tin 1 

 primary mciistcm, consists merely of simple ' meri- 

 stem ' cells. A little higher up the meristem differ- 

 entiates into three layers the dennatogen, the 

 periblem, and the pleroine. These undergo further 

 differentiation, so that a mature root constate of the 

 following parts : ( 1 ) an outer layer, the epidermis, 

 develop! fniin the dernmtogen ; (2) several layers 

 <>f simple cells, the cortex, developed from the peri- 

 blem ; (3) the fibro-vascular cylinder, an inner- 

 most mass of simple cells, and the pith, l>oth 

 developed from the plerome. The root-cap serves 

 as a protection to the delicate growing-point of the 

 root as it forces its way through the soil. It is 

 continuously worn away by the particles of the soil, 

 and as continuously reformed by the primary meri- 

 stem cells. The epidermis binds the whole to- 

 gether, and many of its cells develop into long 

 hairs, the almorbing root-hairs already mentioned. 

 The cortex has no special function. The bundle 

 sheath, when it occurs, separates the cortex 

 sharply from the rest of the root. The fibro- 

 vascnlar cylinder is the part of the root that acts 

 as the channel for the ascending water and salU 

 {crude sap), and gives it also its strength as an 

 organ of attachment. The cylinder consists of 

 smaller bundles of wood vessels and of bast fibres 

 arranged alternately. The centre of a young root 

 is occupied by pith. 



Secotii/'iiif thickening may occur in root* as in 

 stems, and is due to the formation of a layer of 

 actively dividing cells, the cambium. The bundles 

 of wood and 1'iist are placed alternately in a 

 cylinder, and the cambium ring seen in transverse 

 section waves in and out so that it passes insiile 

 the bast bundles and outside the wood bundles. 

 The result of this is that the secondary thickening 

 of roots results in a mass of tissue similar to that 

 formed by secondary thickening of steins (see 

 STEM). Thickened roots usually lose their ori- 

 ginal cortex and epidermis and gain a fresh cover- 

 ing in the following way. A layer of ti-sue, the 

 root-sheath or endodermis, is differentiated which 

 envelops the fibre- vascular cylinder ; within this 

 lies a layer of parenchyma, simple tissue ; it is 

 called the pericainbium or phellogen. This layer 

 gives rise to the new covering, which consists of 

 two parts, phelloderm and periderm (which is of 

 cork), corresponding to the original cortex and 

 epidermis. 



Forms of Roots of Seed Plants. The primary 

 root is merely the enlarged radicle of the seedling ; 

 it is the direct prolongation of the stem. All 

 secondary roots arise from thin first root; the 

 secondary roots may give rise to others and so on 

 until the common much -branched root is formed. 

 If the primary root is very thin the whole system 

 is called a fasciculated root ; if the secondary fibres 

 are also very fine it is called a fibrous root (e.g. 

 the grasses) ; if the fibres are very much thickened 

 it is said to be a tuberous root (e.g. the dahlia). 

 When the primary root is much thicker than the 

 secondary roots it is called a tap-root, and may lie 

 further classified according to its shape, as spindle- 

 shaped (e.g. the carrot) or turnip-shaped. Mucli- 

 thickened root* are usually storehouses of food 

 material e.g. the fasciculated root of the Turk's 

 cap, lily, or the tap-root of the turnip. Root* are 

 also described as fleshy (e.g. the beet-root), or as 

 woody (e.g. the roots of trees). Fleshy roots 

 usually die in the autumn ; woody roots may live 

 for many years, even when the shoots die down 

 every autumn. Knot* are usually buried in the soil, 

 bat they may be aerial, as for instance the roots of 



climhing plants such its ivy; these arise from the 

 stem ami li \ the plants to 'their supports. In the 

 tropics many plants have aerial root* ; thus the 

 mangrove forms forests in the swamps. The Indian 

 fig has aerial roots from which, after they have 

 Income fixed in the ground, new plants may spring. 

 < lichids are examples of the Epiphytes, which have 

 aerial roots widen merely fix the plants to trees 

 but do not derive any nourishment from them. 

 Aquatic plants often have roots which do not pene- 

 trate into the mud but float freely in the water. 

 Many plants will form new roots after all the old 

 ones have been cut oil' if they are placed in water 

 or in a damp soil. This is taken advantage of bv 

 gardeners in their method of propagating sucli 

 plants as fuchsias, oleanders, <Xre. from cuttings. 



Esculent roots an- numerous, and many roots also 

 contain secretions either peculiar to themselves, or 

 more abundant than in the other parts of the plant, 

 and become therefore useful in medicine or in the 

 arts, while some are very ivoisonous. The roots used 

 for food, l>csides the Tul>er (q.v.), Bulb (q.v.), 

 and Conn (q.v.), are generally those which are 

 thick and fleshy. The plants to which they belong 

 are of very different genera and orders some of 

 the natural order Cruciferse, as the turnip and 

 others of the genus Brassica some of the order 

 Chenopodiaceae, as beet and mangold wur/el 

 some of the order Umbelliferiv, as the carrot, 

 parsnip, &c. and some of the order LagnninaMB, 



Comjmmtire. Most of the Thallophytes (Algie, 

 Kungi, and Lichens) have only a rudimcn: 

 differentiation into stem, leaf, and root, or none at 

 all ; but the most highly developed member) of 

 each division have an outward distinction of parts 

 to which such terms may l>e given; still the roois 

 even of these are distinguishable from the roots of 

 vascular plants by the absence of a root -cap, and 

 by their branching, which is never endogenous. 



The Vascular Cryptogams ferns, horsetails, lyco- 

 pods, &c.- are highly dill'crpiitiated in form and 

 in their tissues. 1 lie roots arise in acropetal succes- 

 sion on the stem, or in many ferns on tne petioles ; 

 their branching is monopial or dichotomons : they 

 are all alike the first root never becomes like a 

 tap-root, the lateral roots arise from the innci 

 layer of the cortex. The roots grow by successive 

 divisions of an apical cell. The Selaginelhe. a group 

 of the Lycopodinea', are remaikalde for having tine 

 roots, but a few have no root-caps on them. The 

 growing-point too is formed by a primary mei istem 

 and not by an apical cell. 



The roots of the s 1 PI. nits, consisting of C.ym- 



nosperms i.e. Cycadea', Conifeive. Taxaceie. and 

 (Jnetacea-, anil of Angiospcrms i.e. Monocotyle- 

 dons and Dicotyledons, have already received a 

 general description : a few points ol comparison 

 may here l>e repeated. The primary root is a con 

 tinuation of the primary stem ; it is directed 

 towards the micropyle of the embryo-sac (see 

 (Hfi.K), and even it is of endogenous origin, its 

 first rudiment being covered by the nearest cell of 

 the suspensor. All the parts of the seed plants, 

 shoots and leaves as well as roots, are distinguished 

 from the parts of all other plants, except the l.yco 

 podiacea 1 , and for a short time the embryos of some 

 Conifene, by having a small-celled primary meri- 

 sti-iii at the growing apices instead of the typical 

 apical cell of many Algic, and of the Mnscinew, 

 ferns, and Horsetails. The root cap i- foimed, 

 not as in the cryptogams, by transverse divisions 

 of an apical cell, but l>y rapid division in the direc- 

 tion of the apex, and in gymnosperms from the ]x i ri- 

 blem, in angiosperms from the dennatogen. Also 

 the first rudiments of lateral roots, shoots, and 

 leaves do not arise from single cells, as in the 

 cryptogams, but from protulieraiices of a few small 

 cells. The formation of a closed cambium ring in 



