208 MM. Garreau and Brauwers on the Roots of Plants. 



From this description it is seen that the exfoliation of the 

 cortical zone varies according to the conditions of humidity and 

 temperature under which the radicles are placed ; but we must 

 recognize, side by side with these causes, that it is also subor- 

 dinate to an individual predisposition, since it does not take 

 place always in the same manner when the subjects are placed 

 in identical conditions. 



In the radicles of wheat, barley, millet, vetches, clover, pease, 

 lentils, hollyhock, buckwheat, &c, it takes place by complete 

 disunion of the cells in the midst of a viscid layer. 



In the poppy, Camelina, black mustard, colza, purslane, 

 chervil, corn-salad, &c, it takes place in the form of a hood, 

 composed of cells but slightly adherent, and impregnated with 

 the viscid substance. 



In (Enanthe Phellandrium the exfoliation takes place in strips 

 composed of epidermal cells, which adhere strongly to the sub- 

 jacent tissue. 



In Glyceria it takes place in the form of a hood composed of 

 very firmly coherent cells; in Lemna the exfoliable cortical 

 layer forms a sheath, which adheres by its base to the hemi- 

 spherical portion of the axis of the radicle ; and it is remarkable 

 that this hood already exists while the radicle is still enclosed 

 in the coleorhiza. If Lemna is examined at the epoch when its 

 radicle is beginning to sprout, this organ will be found in the 

 form of a little cylinder, of a darker colour than the surrounding 

 tissue, and it is contained in a groove existing in the inferior 

 surface of the leaf. This cylinder, now measuring about the 

 fourth of a millimetre, is covered by a membrane in the shape 

 of a sheath, composed of cells contiguous to those which bound 

 the inferior surface of the leaf (coleorhiza) . 



When the whole is compressed gradually between glass, the 

 sheath is seen to burst at its apex, and allow the exit of the 

 radicle already enveloped in that persistent hood which is ob- 

 served at the extremity of the root when this is examined in the 

 adult condition*. This layer or hood, which, as we have said, 

 adheres to the apex of the radical axis, grows for a long time 

 after the radicle has ruptured the summit of the coleorhiza ; for 



the soil, or to the sides of earthen pots where these are kept moist. — 

 A. H. 



* This firm hood-like body, the pileorhiza, placed on the point of the 

 root like the head upon an arrow, seems to occur on all nascent adventitious 

 roots, before they have broken out from the cortical layer of the stem. 

 Originating in the cambium-region, they push the cortical or epidermal 

 tissue before them, and absolutely rupture the latter, the ragged edges of 

 which stand up round the base like a collar, forming the so-called coleo- 

 rhiza. — A. H. 



