370 



CASSELL'S POPULAR GARDENING. 



any case must te aceompanied by an adequate supply 

 of moisture and of nutritive material, the latter, as 

 we have seen, having heen accumulated under the 

 influence of light. 



S^welling and Shrinking. — Growth is in- 

 separahly connected with a swollen or turgid -condi- 

 tion of the cell, due to the afflux of liquid matter 

 and to the elasticity and extensibility of the cell- 

 walls. Hence anything that promotes turgescence in 

 so far favours growth, and anything which checks 

 or prevents it has 

 a restraining in- 

 fluence on gi'owth. , 

 A withered shoot, 

 one in which eva- 

 poration has been 

 in excess of the 

 supply, ceases to 

 grow — it shrinlss, 

 receives a check as 

 gardeners say, • and 

 it may or may not 

 revive when fresh 

 supplies are forth- 

 coming. 



Effect of Pres- 

 sure. — Pressure 

 also necessarily 

 checks growth by 

 the resistance it 

 affords to expan- 

 sion. A branch of 

 a tree fastened 

 tightly against a 

 wall will naturally 

 grow much less on 



the side next the wall. The trunk of a tree tightly 

 tied up to a stake will increase less in proportion 

 than the free part above the tie, where the trunk and 

 branches have fair play. This is one reason why 

 trees that have been too tightly or too long staked 

 are apt to have their heads blown off or injured by 

 the wind. Not only is there thus a greater leverage 

 on the head, but the confined stem below is less 

 Able to resist the strain. 



The benefit resulting from incising the bark of 

 ■'' hide-bound " trees may likewise be explained by the 

 lessened pressure so o'btained, and the free growth of 

 trees such as the Plane may be explained by the 

 shedding of large slabs of the outer bark, or the deep 

 cracks and furrows in the bark of other trees, which 

 diminish the resistance offered by the bark to the 

 growing tissues underneath, thus enabling them to 

 grow faster and further. 



F)g. 53.— Shows a " Procumbent " Stem, in which the Stem is at first acted 

 on by grEiTitation, and a.(terwards ascends towards the light by the 

 agency of unequal growth on the two sides. 



Direction of G-rowth.— The direction in which 

 growth takes place depends partly on external con- 

 ditions, partly on internal inborn circumstances, 

 handed down from plant to plant as its inheritance 

 from its predecessors. Thus, if the food-supply and 

 the illumination be equal on all sides, we may expect 

 a spherical form to result ; while if the conditions 

 act more powerfully in certain directions than in 

 others, we may expect growth in more or less verti- 

 cal lines, as in the case of the stems of trees, or in flat 

 planes, as in the case of leaves ; and there will be all 



kinds of interme- 

 diate forms accord- 

 ing to the varying 

 circumstances or 

 varying combina- 

 tions of circum- 

 stances. The down- 

 ward tendencies ex- 

 hibited by roots in 

 their growth when 

 unfettered by ham- 

 pering obstacles 

 have been shown 

 to be due to the 

 action of gravita- 

 tion, for even when 

 exposed to light or 

 moisture as in Figs. 

 51 and .52, the roots 

 are not turned 

 from their coui-se. 

 The cause of the 

 upward tendency 

 exhibited by the 

 stem, see Figs. 52 

 and 53, though con- 

 nected with the 

 necessity for exposing the leaves to light and air, is 

 not understood, and such terms as " geotropism,'' 

 "negative geotropism," " heliotropism," "hydro- 

 tropism," * and the like, now frequently used, arc 

 mere expressions of phenomena, and not explana- 

 tions of them. 



In the accompanying illustrations the varying 

 direction of jjlants is exemplified. In Fig. 51 the 

 roots are seen descending by gravitation; though 

 exposed to hght from below, they neither bend 

 towards nor from it. In Fig. 52, where the Beans arc 

 supposed to be planted in damp moss, the roots still 

 descend away from their supplies of moisture, thus 



• Heliotropism— the tendency of growing parts to turn to 

 the sun ; geotropism— the tendency to turn towards the 

 earth ; hydrotropism— the tendency to turn towards mois- 

 ture. The reverse conditions are implied by the prefli 

 "negative." 



