7 o 



a new growth of root-hairs enables them to become closely- 

 attached to the particles of the soil, and in this case it is ad- 

 visable to remove the leaves. A plant also growing in a dry 

 soil as a habitat, if the season be unusually dry, gives out 

 more fluid than it takes up, and soon dies. The plant, in ordi- 

 nary conditions, usually takes up rather more than enough to 

 supply the demand of transpiration, as it has also to perform the 

 processes of growth. Weeds, also, when removed from the soil, 

 rapidly fade and die from loss of water by transpiration. That 

 they do not die merely from exposure of the roots is clear ; if so, 

 how could we explain the cases, such as we have presently to no- 

 tice, of the withering of shoots or flowers which have been cut off 

 from the plant ? The conditions of withering are especially of 

 interest to those who are cultivators of vines in this country, since 

 the roots are generally outside and the stem and leaves inside the 

 hothouse. If the leaves in the warm atmosphere within give off 

 a large quantity of water, while the temperature is very low out- 

 side, the plant will not be supplied with sufficient moisture, 

 because the root can take up moisture only after a certain 

 degree of temperature has been reached. 



The aerial parts of plants, as stems and leaves, are not capable 

 of absorbing watery vapour from a moist atmosphere, or from 

 water poured over them. We might expect that the upper 

 surface of the leaf being exposed to rain could absorb some, 

 but it does not ; — the upper surface of the leaf is not usually 

 provided with stomata, and is covered with a continuous cuticle, 

 often covered in turn with wax, and so rendered waterproof. 

 The aerial roots of Orchids, which hang down into the air, may 

 draw watery vapour from it, as in those forms which rest on other 

 plants for support, but obtain no nutriment from them, and are 

 known to botanists as epiphytes or air plants. The roots of 

 these plants, which are peculiarly adapted for taking up mois- 

 ture from the air, are, as a rule, invested with a very special 

 covering, known as the velamen, which consists of several layers 

 of spirally thickened epidermal cells, often quite empty, which 

 communicate freely with the external medium, and abut on the 

 cells of the interior of the root. These velamen cells act in the 



