Respiration. 561 



the greatest care has been exercised in the removal to secure 

 all the roots without injury, and when abundance of water is 

 within their reach. This may arise from a paucity of small 

 roots ; and when a tree has but a few large coarse roots, it is 

 almost impossible to lift it with a ball of earth ; and a very short 

 exposure of the fibrils to a drying wind will destroy their delicate 

 spongioles or absorbing parts. Some plants will readily replace 

 their fibrils, whilst others, and especially those with coarse roots, 

 do so with difficulty, or not at all, if the circumstances be un- 

 favourable. In instances of this kind it is useless to confine 

 one's attentions to the roots. The top must be cared for, and 

 this is best done by covering it over with bast mats in very hot 

 sunny or drying weather. From time to time the mats should 

 be moistened to moderate the transpiration of the plant. The 

 trouble involved in this operation would be amply repaid by 

 the preservation of choice shrubs* 



After a prolonged drought the leaves of many herbaceous 

 plants, whose roots do- not penetrate far into the soil, droop 

 during the day when exposed to the fierce rays of the sun, 

 because the evaporation is in excess of the absorption. But 

 the dews of night are sufficient, as a rule, in our climate, to 

 afford them moisture enough to rise again and recover their 

 former firmness. In hotter, drier climates, however, as in 

 some parts of tropical and sub-tropical Africa, a great number 

 of plants succumb, that would otherwise have enjoyed a much 

 longer existence. In dull, rainy weather the amount of evapo- 

 ration is almost nil. 



From the differences in their structure, plants possess greater 

 or less power of resisting the effects of continued dry weather. 

 These differences are in texture, number of stomates, thickness 

 of epidermis, and also depth reached by the roots. Relatively, 

 annual and perennial herbs suffer more from drought than 

 woody and arborescent plants; and aquatic plants wither 

 almost directly they are taken from the water. Trees with 

 coriaceous leaves, like the common Box, with hard wood or 

 deep roots, suffer little from long droughts; and the same 

 may be said of most plants with succulent, fleshy leaves or 

 stems, owing to their thick epidermis and few stomates. The 

 plants belonging to the latter category abound in hot, dry 

 countries. 



The condensation of the sap in the tissues of the leaves is 

 insufficient of itself to give it the necessary nutritive pro- 

 CD a 



