26a 



THK TROPICAL AQRiCULTVUiBf. 



[O&r. 1, J 



passes fioni the roots througE the cells of the plant, 

 and is so earned up to tlie leaves, kc. AVater may 

 also be takeu in by the leaves, as we know is the 

 case when they are cut off and placed on the surface 

 of water; also one use of a very damp atmosphere 

 in some of our houses for young plants is to supply 

 them with water, or at least not to allow the water 

 already in the leaves to drain away by evaporation. 

 We might just note here that roots seem to have 

 the power of turning towards moisture or to sub- 

 stances they seem to like to grow in, where also 

 they are generally found emitting more root branches. 

 We learn also that roots have the power to secrete 

 fluids [when in contact with iJarticles of] the soil ; 

 at times these secreted fluids are said to aid in dis- 

 solving solid matter (as marble even) which may be 

 taken into their tissues when so dissolved. Some 

 roots store up starchy or other matter for the future 

 use of the plant. We may see the swollen growth 

 of the preceding year decaying away as the leaves 

 form, and another reservoir for such starchy matter 

 is built up. There are good examples of this to be 

 met with among bicinials, as many Campanulas with 

 a thickly developed "first-root" or tap-root. Dahlia 

 also is a good example, and I expect that Oxalis 

 hirta has its root developed for a similar purpose. 

 Potatoes are about the best example one knows, how- 

 ever, and it is a really marvellous sight to see the 

 innumerable starch granules when exuded from a cut 

 piece, under a powerful microscope ; but we have 

 considered Potatoes as branching from (underground) 

 stems, though capable of emitting root fibres as well 

 as buds ; they also absorb fluid matter through their 

 exterior which bulbs and corms of Hyacinth and 

 Gladiolus are, I should imagine, unable to do, by 

 reason of their " coat." We might by " stretching 

 a point" consider that roots, as the word is understood 

 by horticulturists, of Drosera, Primula (and Dodeca- 

 theony), had another use to perform, that is, to 

 continue the species by budding. 



The Manner in which Roots Perform their 

 Work. — Bearing in mind the remarks made as to 

 the structure especially, but also of the external 

 features of roots, let us discuss how they perform 

 their varied works in aiding the life of the plant. 

 1. It is not difficult to see how they act as anchors 

 or supports. The few fibrous roots noticed in the 

 Pjnguicula and Drosera, the swollen tap-roots of 

 Astragalus or Hippocrepis, the huge branching roots 

 of the Oak or Elm, all plainly tell us by their con- 

 struction and direction that they support or fix the 

 growths above them. Often the roots of a tree will 

 extend underground as far as the branches do above. 

 There seems a mutual dependence of one upon the 

 oiher, causing them to grow more profusely, or 

 stretch for further distances as required, 2. As to the 

 absorbing of food by roots. We have seen that tlie 

 external tissue of all young roots is soft, and that 

 this jouug tissue, either of cells alone, or of hairs 

 and cells, is present everywhere in roots, from the 

 moss to the highest tree, As any soft substance will 

 absorb water when placed in it, so we might assume 

 the soft tissue of the root would do wo- We can 

 indeed see coloured water entering into the root- 

 hairs when these are placed in it. l^Iost plants are 

 not surrounded by very evident supplitb oi' water, 

 and it is here that the longer root-hairs become 

 tnost useful. They stretch among the particles of 

 soil. an(l abstract the watery films which we learn 

 enclose all but the very driest particles, All the 

 solid food elements needed, if dissolved in water, 

 would thus be able to enter into the plant. We 

 have next to see how such are taken from the root 

 to the leaves, where we know the fluids undergo 

 necessary changes upon their distribution throughout 

 the plant. If we consider— 1st, the action of a 

 sponge or piece of sugar when placed in water; and 

 2nd, the effect produced by sucking the air from a 

 ■straw held in water, we shall have a partial clue 

 to the modus operandi. 



C'lpillaiy Action and Pressure of the Air.— 

 We know that in both the cases referred to water 

 tiill rise above its level in the dish iu which eugu 



and straw were placet!. This rising of fluid is due 

 to simple physical laws, depending in one case on 

 the closeness of the particles of sugar for its effect, 

 the water rising up the thread-like openings bet- 

 ween the particles, and in the other on the partial 

 vacuum which would be caused by suction necessit- 

 ating that the water should rise [by atmospheric 

 pressure]. 



Osmosis. —The cells of a plant, as we saw, are so 

 arranged that watery fluids would rise in or between 

 them as in the sugar, and would be much helped by 

 the evaporation going on from the leaves, compell- 

 ing its rise, as the suction of water up a straw does. 

 But we often have, as in winter-pruned Ivy, a great 

 surface to be supplied with nutriment, and no leaves 

 to aid in drawing fluids up the stem. Clearly we 

 have great need for another process by which it 

 shall rise, and this has been found by botanists to 

 be indeutical with a well-known physical process 

 termed osiiiosis. Roughly this proce^^s of osmosis 

 takes place wherever a denser fluid is separ- 

 ated from a lighter one by any thin-walled sub- 

 stance such as parchment, when a current vrill be 

 set up from one to the other until the densities of 

 the fluid on either side are similar. AYith the cell 

 wall instead of parchment the cell's denser contents 

 might pass out into the earth, whilst water would 

 be takeu in were it not that the evaporation from 

 the surfaces of the plant keeps up a continual inflowing, 

 whilst the currents from one part of the plant to the 

 other are stimulated by the denser contents of cells 

 from es'aporation takes place, dra^ving lighter fluids 

 towards them. This ascent of sap is naturally much 

 more vigorous and vice rersd in summer, aud nearly 

 ceases in wiuter. Experiments have shown its force 

 to be very considerable. 



Stem Cuttings .a^jd their Roots. 



These are mostly fibrous. At times, however, some 

 are specially developed, and grow much in length, 

 and harden very considerably. 



If we place a suitable piece of stem in the earth, 

 so that it produces roots, various changes take place 

 at the cut end, as follows : — ■ 



1. The pith cells shrink slightly and become dis- 

 coloured, owing to the decay of the damaged cells. 



2. The cells forming the ring around the pith cells 

 grow so as to produce a ring of warty structure. 

 This is called "the callus." 



3. From below this ring or series of worthy growths 

 are given off in an ordinary manner a greater or less 

 number of roots. In some cases these roots pene- 

 trate through the warty cells, but generally I have 

 observed that they spring from below them. 



Probable Reasons For These Changes oi Tissue. 



1. We can understand that the pith cells should 

 decay back where broken — possibly they harden a 

 little— in some cases where a stem is cut at both ends 

 one end may dry. 



2. A^'e see the necessity for the woody aud fibro 

 vascular cells to be sealed up b}- some means — na 

 they are by the callus — or else the sap iu them 

 would not be properly fubjeoted to the influence 

 which would cause it to spread through the cutting. 



0. We have seen that in all root-branches or root> 

 such as these are that there is necessity for some of 

 the denser central tissue entering into their structure. 

 We should then expect that they would (as we 

 saw in Strelitzia) start from the thickened layer of 

 cells. These roots push through the surrounding cells 

 in the ordiuary manner. 



I believe that before much or any root-growth 

 takes place there must be some pressure of the fluid 

 causing tension in the cells of the plant, aud this 

 state cauuot well be if they are open iu the ground 

 or " bleeding." 



A knowledge of the preceding facts will, I think, 

 show us why in many cases we should not expect 

 cuttings to strike. 



What will Striki;, 



1. A\'e should not expect any stem destitute of a 

 fair band of the harder (fibre vascular) cells, pither 

 to " callus " or subsequently to emit loote, 



