and Exfoliation in the Roots of Plants. 207 



supposition that this kind of development occurs in the gene- 

 rality of plants. Very frequently, when the medium in which 

 they vegetate stands at a rather low temperature, the extremities 

 of the radicles on the fibrils are tardy in exfoliating ; and then 

 their elementary cells, instead of becoming detached singly, ex- 

 foliate in sheets like an epidermis, strips of which they in 

 fact are. 



The seed of black mustard, germinating at 54° F., produces 

 radicles in which the cells of the apex of the axis, and those of 

 the exfohable layer which clothe it, are filled with granules 

 whose opacity renders the most persevering investigation fruit- 

 less. But if we wait until adventitious roots are formed, we 

 may find among these some not more than a fifth of a milli- 

 metre in diameter, and perfectly transparent. A fibre of this 

 kind placed on a slide shows, without the necessity of injuring 

 it, its axis with its spiral vessels and its cortical layer, which, 

 instead of exfoliating like that of wheat, becomes detached in 

 the form of a cap, formed by the union of several superimposed 

 layers. A remarkable point is, that these caducous layers 

 cease to be so when the radicle and cotyledon are kept in a 

 sufficiently moist atmosphere, if the saturated atmosphere is 

 only allowed access to the apex of the root. In that case, we 

 see the most external cells of the cortical layer emit absorbent 

 appendices, like those on the epidermis of the base and middle 

 portion of the root, and the spiral vessels, which in ordinary cases 

 terminate at a certain distance from the apex of the radical axis, 

 present themselves quite close to the extreme limit of this 

 region, which shows that there exists an intimate correlation 

 between the functions of the absorbing appendices and those 

 of these vessels, as was supposed by Link. This faculty pos- 

 sessed by the epidermal cells, of emitting absorbent appen- 

 dices to counteract the deficient supply of water or humidity, has 

 a no less remarkable influence upon the direction of the radicle. 



If we moisten the meshes of a sieve with distilled water con- 

 taining a trace of chloride of calcium, so that the cloth may not 

 become completely dry in the open air, and then scatter over 

 the outside of the cloth seeds of Camelina, the latter will adhere 

 readily on account of the mucilaginous film they form when 

 brought in contact with water. If the inside of the cloth of the 

 sieve is then covered with a thick layer of Swedish filtering- 

 paper saturated with water, the seeds germinate, and their 

 radicles, instead of taking a direction perpendicular to the 

 horizon, creep along the outside surface of the cloth, and re- 

 main attached to it by the aid of their absorbent appendages*. 



* In like manner we often find an extensive felted mass of fine radical 

 fibres adhering firmly to pieces of bone, shell, or porous stone buried iu 



