THE ROOT-CAP. 107 



Thus in most cases the group composing the point of growth 

 consists of three liinds of superposed ctills, so arranged in la3ers 

 that each gives rise to a determinate portion of the foi-ming 

 root : (1) the outer or lower laj-er, to the root-cap and the rest of 

 the epidermis; (2) the middle, to the cells wliich are immediately 

 under the epidermis, — the cortex ; (3) the inner or upper layer, 

 to the central cylinder. But in some plants ■■ there are more 

 than three layers of initial colls (e. g., Sparganium, Eaphanus, 

 etc.), while in others there are less than three (e. (;., only one in 

 CucurbitacefE, two in Triticum). 



322. The Root-cap. The growing-points of nascent roots origi- 

 nate just below the surface of the organ whence they proceed ; 

 hence roots are said to be formed endogenousl^y. In emerging, 

 the}' rupture the la^er of tissue bj- which they had been covered, 

 but are from the first protected at the end b}' a thicker or thinner 

 mass of parenchyma, — tlie root-cap.^ 



323. It does not alwa_ys have the same origin, as will be seen 

 bj' the notes,' nor has it the same shape and size in all plants. 



^ Janczewski (Ann. ties Sc. nat., ser. 5, tome xx., 1874) describes six types 

 of developmpnt of the tissues of the root : — 



1. Foiiv distinct layers of meristem ; namely, Plerom, Periblem, Dei-mato- 

 gen, and Calypti'ogen ; e. g., Ili/drocharis. 



2. A distinct Plerom and Calyptrogeu, but the Periblem and Dermatogeu. 

 have initial cells in common ; e. (/., Graminece. 



3. A distinct Plerom ; the Periblem, Dermatogen, and Calyptrogen have 

 common initial cells ; e. g., Iridaccce. 



4. A distinct Plerom and Periblem ; the Dermatogen and Calyptrogen have 

 common initial cells ; e. g., Hdianthus animus. 



5. All four layers have common initial cells ; e. g., Phaseolus and Fisum. 



6. Only a distinct Plerom and Periblem ; therefore there is neither true 

 epidermis nor root-cap, since these are formed simply by outer layers of the 

 Periblem; e. g., Gijmnospcrmce. 



Treub (1876) and Eriksson (1878) distinguish seven types. 



2 According to Olivier, u part of tlie tissue thus broken through by the 

 advancing radicle of grasses remains at its base, as the coleorhiza, while the 

 rest becomes the root-cap (Ann. des Sc. nat., ser 6, tome xi., 1881, p. 19). 



■• According to Flahault (Reeherches sur I'accroissement tenninal de la raeine 

 chez les Phaneroganies, Ann. des Sc. nat., ser. 6, tome vi., 1878), who bases his 

 opinion on an examination of three hundred and fifty species of Phaenogams, 

 tlie terminal growth of the root may be referred to two structural types which 

 are characteristic of monocotyledons and dicotyledons. 



In monocotyledons the epidermis is generally formed by the initial cells of 

 the cortex. The epidermis never gives rise to a root-cap ; the root-cap once 

 formed is continually renewed by the activity of its internal layers. In dicoty- 

 ledons, on the other hand, the epidermis is almost always independent of the 

 cortex ; the root-cap is continually renewed by the activity of the cortex and 

 epidermis. 



