EKKECTS OF GUOWINU C15LL.S UN EN VIUONMEN'T. 513 



a riipid increase of heat, provideil, of course, that the other factor of growth pre- 

 viously iudicated, viz. water, is present in suthcieut quantity. 



It has been shown iu a previous section that tlie mineral salts, which plants 

 require for the production of building materials, are brought by means of water 

 to the place of need, and that this transporting water is raised from below by 

 evaporation from the surface of organs exposed to the air and sun. This evapo- 

 ration, however, requires much warmth, and tliere can be no doubt that the 

 hastened or retarded development of vegetation is partly dependent on quickened 

 or retarded transpiration; that is to say, on the greater or less amount of heat 

 supplied. The couduction of food-.salts by means of water from the soil is, how- 

 ever, not by any means growth; it is only a preparatory' process, as also is the 

 formation of organic materials in gr^en cells, and the complicated transformations 

 and distribution of materials which follow the elevation of the water from the 

 ground. Warmth is an essential condition of that process which is being here 

 discussed, that is of growth in its narrow sen.se, as well as of all these preparatory 

 processes. 



The part taken by heat in actual growth, that is, in the transformation of fluid 

 building materials into firm, organized portions of the plant-body, and increase 

 of bulk of the cells, cannot be essentially different from that which occurs in 

 other molecular re-aiTangements and chemical changes. Heat, according to pre- 

 valent theories, is the expression of vibration of ultimate particles. Those vibra- 

 tions of ether which are known as free heat can induce a corresponding motility 

 of the molecules in any ponderable body. Similarly, heat induces a state of 

 motility amongst the molecules of living protoplasm. We nuist imagine that 

 work is done upon the organic bodies which constitute the building materials 

 of plants, that they are led in a fluid state to the regions where they are required, 

 and there transformed into solid organized matter. In this way free heat is 

 transformed into latent heat, and in this sense we may regard growth as a con- 

 sumption of free heat. Accompanying this organizing action of heat there is an 

 insertion of new molecules between the pre-existing ones. The separation of these 

 latter is of course brought about, as already described, by turgidity. Thus, by 

 the co-operation of heat and turgidity, fluid organic materials are changed into 

 firm, solid, organized substances, and in this way the organized portions inci-ease 

 in bulk, in other words, they grow. 



EFFECTS OF GEOWING CELLS ON ENVIRONMENT. 



Work is not only performed in the interior of cells, but pressures also come 

 into action which operate on the surroundings with irresistible power. What 

 the cells, apparently so delicate, are able to perform, borders almost on the in- 

 credible. 



Where the filamentous hyphal threads of crustaceous lichens have penetrated 



VOL. I. 33 



