THE ROOT-CAP. 107 



Thus in most cases the group composing the point of growth 

 consists of three kinds of superposed colls, so arranged in layers 

 that each gives rise to a determinate portion of the forming 

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

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

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

 to the central cylinder. But in some plants 1 there are more 

 than three layers of initial cells (e. </., Sparganium, Raphanus, 

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

 Cucurbitaceae, two in Triticum). 



3*22. 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 endogenously. In emerging, 

 the}' rupture the layer of tissue by which they had been covered, 

 but are from the first protected at the end by a thicker or thinner 

 mass of parenchyma, - - the root-cap. 2 



3*23. It does not always have the same origin, as will be seen 

 by the notes, 3 nor has it the same shape and size in all plants. 



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

 of development of the tissues of the root : - 



1. Four distinct layers of meristem ; namely, Plerom, Periblem, Derraato- 

 gen, and Calyptrogen ; c. y., Hydrocliaris. 



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

 have initial cells in common ; c. y., Grinnim". 



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

 common initial cells ; c. y., Iridncece. 



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

 common initial cells ; c. y., Hclianthus unnuus. 



5. All four layers have common initial cells ; e. y., Phaseolus and Pisum. 



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 

 Peril ilem ; c. g., Gymnospcrmce. 



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



2 According to Olivier, a part of the 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). 



3 According to Flahault (Recherches stir 1'accroissement terminal de la racine 

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

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

 the 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. 



