86 



CASSELL'S POPTJLAE GARDENING. 



problem not yet completely solved ; how it does so is 

 more apparent, and the results are of course obvious. 

 The reasons for its descent are to be found in 

 the circumstances by which it is surrounded, by 

 its structm-e, and by its mode of growth. Gravi- 

 tation has been shown by Thomas Andrew Knight 

 and many subsequent observers to influence the 

 downward course of roots, or rather, it has been 

 proved that where the effects of gravitation are 

 counteracted, the roots do not grow downwards. 

 Knight sowed seeds on the outer side of a wheel 

 kept rotating in a vertical direction. By these means 

 the force of gravitation was overcome or replaced by 

 centrifugal force, or the tendency to separate or fly 

 off from a revolving body. Under these circumstances 

 the roots always turn outwards, away from the centre 

 of the wheel (Fig. 13). When the seeds are sown 

 on a horizontally moving wheel, the roots assume an 

 intermediate position between 

 horizontal and vertical, as the 

 result of a compromise between 

 ' the tendency of gravitation to 

 pull them down, and the tend- 

 ency of centrifugal force to keep 

 them horizontal (Fig. 14). The 

 quicker the rotation of the 

 wheel, .the more horizontal the 

 direction of the root. 



The presence of air is es- 

 sential to the roots, and also 



the existence of moisture, provided that it be not - 

 stagnant or excessive. Then it is well ascertained 

 that the growth in the leng-th of roots occurs chiefly 

 in one particular part of the root, and that the 

 portion just above the root-cap or shield before 

 mentioned. 



How the Radicle Penetrates the Soil — 

 Movement of the Roots. — The conical point of 

 the root, thus protected, is thrust forward, or rather 

 downward, into the soil by the lengthening that 

 takes place just above it, the body of the seed and 

 the superincumbent soil acting as the " fulcrum," or 

 point of support. 



And now comes into operation a very marvellous 

 process, only lately brought to light, chiefly bj' the 

 patient experiments of Darwin. It may be premised 

 that all the growing parts of plants are endowed 

 with a power of movement, now in this direction, 

 now in that, and varying in direction and amount 

 according to circumstances. These movements are 

 not readily visible to the naked eye, except in some 

 marked cases, but by various contrivances it is an 

 easy matter to prove the fact. For the moment we 

 confine ourselves to the movements observed in the 

 radicles of seedling plants. 



Tig. 12 Germination 



couts are torn, tlie 

 and. tlie arcli of tlie 



A short hair-like thread of glass, with a minute 

 piece of black sealing-wax, was attached to tho 

 radicle by means of shellac in solution. By its side 

 was placed a stick bearing a card with a black dot 

 upon it. Then, by observing at stated intervals 

 imder a magnifying-glass the relative position of the 

 fixed dot, and of the bead affixed to the radicle, the 

 movements were rendered , apparent, and, indeed, 

 were "plotted " out as on a map. In some cases the 

 radicles were allowed to grow over plates of smoked 

 glass. As the tips of the roots moved they left a 

 track in the soot by which the glass was covered, 

 and their movements were thus rendered apparent. 



It may then be taken as proved that one means by 

 which the radicle penetrates the soil is by virtue of 

 the power of rotary movement with which it, like all 

 the other growing parts of plants, is endowed, and 

 by means of which it penetrates the soil as the 

 point of a cork-screw enters 

 a cork. The pressure of the 

 soil above and the weight of the 

 seed give the requisite bearing, 

 such as is afforded in the case 

 of the screw by the pressure 

 of the hand. Many radicles 

 are invested with a, fringe or 

 sheath of fine " root - hairs," 

 which gTOw out from the sm-- 

 face of the root and give that 

 cobweb-like appearance which 

 is so commonly seen when roots have access to mois- 

 ture or to rich food. These hairs, of which we shall 

 have to make mention later on, help to keep the 

 root in position; and, moreover, they are endowed 

 with the same gyratory motion as other gfowing 

 parts. While increasing in length, the root in- 

 creases in thickness also, especially above, so that its 

 wedge-like or conical form is increased, and the 

 force exerted by the growing root constantly in- 

 creased ; hence roots act, as Darwin says, " like a 

 wedge of wood, which, whilst slowly driven into a 

 crevice, continually expands at the same time by the 

 absorption of water, and a wedge thus acting will 

 split even a mass of rock." 



This power of movement depends probably upon 

 variations in the. currents of liquid passing from cell 

 to cell, causing turgidity now of this part of the 

 root, now of that, and on the periodic passage from 

 one cell to another of its contents; matters which 

 will be spoken of later on. 



Sensitiveness of Roots. — Before quitting the 

 subject of the penetration of the soil by the root, 

 it is necessary to allude to another marvellous pro- 

 perty possessed by the growing tips of the roots, and 

 exercised by them, in conjunction with the power of 



of Bean. The seed- 

 radicle is protruded, 

 caulicle is visible. 



