OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 99 



that, if the succulent portions be dried up, their vital properties are de- 

 stroyed. Thus it is in the soft tissue at the extremities of the radicles 

 or root-fibres, that the function of absorption takes place with the 

 greatest activity ; so that these parts have received the name of spon- 

 gioles: it is in the cells which form the soft parenchyma of the leaves, 

 that the elaboration of the sap takes place, the fixation of carbon from 

 the atmosphere, and the preparation of the peculiar secretions of the 

 plant : and it is in the space between the bark and the wood, which is 

 occupied (at the season of most active growth) by a saccharine glutinous 

 fluid, that the formation of the new layers of wood and bark takes place. 

 Now, as soon as these parts become consolidated, they cease to perform 

 any active vital operations. The spongioles, by the lengthening of the 

 root-fibres, become converted into a portion of those fibres, and remain 

 subservient merely to the transmission of the fluids absorbed; the leaves 

 gradually become choked by the saline and earthy particles contained 

 in the ascending sap, which they have had no power of excreting, and 

 they wither, die, and fall off; and the new layers of wood and bark, 

 when once formed, undergo but little further change, and are subser- 

 vient to little else than the transmission of the ascending and descending 

 sap to the parts where they are to be respectively appropriated. 



152. There are some remarkable instances in both the Animal and 

 Vegetable kingdoms, of an immense preponderance in the amount of 

 the fluids over that of the solids of the structure. This is characteristic 

 of the whole class of Acaleplice or Jelly-Fish, giving to their tissues that 

 softness from which their common name is derived ; these animals, in 

 consequence, are unable to live out of water ; for when they are removed 

 from it, a drain of their fluids commences, which soon reduces their 

 weight to a degree that destroys their lives, a Medusa weighing fifty 

 pounds being thus dried down to a weight of as many grains. The most 

 remarkable instances of a parallel kind among Plants, are to be found 

 in the tribe of Fungi ; certain members of which are distinguished by 

 an almost equally small proportion of solid materials in their textures, 

 presenting a most delicate gossamer-like appearance to the eye, and 

 possessing such little durability, that they come to maturity and undergo 

 decay in the course of a few hours. These are not inhabitants of the 

 water, but will vegetate only in a very damp atmosphere. 



153. As we find various Plants and Animals very differently con- 

 structed, in regard to the amount of fluid contained in their tissues, so 

 do we also find them dependent in very different degrees upon a con- 

 stant supply of external moisture. There is no relation, however, be- 

 tween the succulence of a plant, and the degree of its dependence upon 

 water ; in fact, we commonly find the most succulent plants growing in 

 the driest situations ; whilst the plants, which are adapted to localities 

 where they can obtain a constant supply of fluid, are not usually re- 

 markable for the amount of water in their own structure. This, how- 

 ever, is easily explained. "We find the most succulent plants, such 

 as the Sedums or Stone-crops of our own country, and the Cacti and 

 Euphorbice of the tropics, in dry exposed situations, where they seem 

 as if they would be utterly destitute of nutriment. The fact is, how- 

 ever, that they lose their fluid by exhalation very slowly, in consequence 



