GROWTH. 333 



sary energy is being evolved, that the temperature is favour- 

 able, and that its cells are turgid, the organ will continue to 

 grow indefinitely. Speaking generally we may say that the 

 power of growing is possessed by an organ (and this is equally 

 true of each individual cell of a multicellular organ) only 

 during a particular period of its life : when this period is past, 

 and during this period the organ has attained its limit of size 

 and its permanent form, growth ceases, however favourable to 

 continued growth the conditions may be. 



Under certain circumstances, however, a mature organ, 

 which has completed its period of growth, may again begin 

 to grow. When, for instance, the general conditions under 

 which an organ has been growing are suddenly altered, the 

 disturbance may stimulate the organ to grow. Thus, it is 

 commonly observed that when the haulms of Grasses are 

 laid horizontally they will, in the course of a few days, assume 

 an erect (vertical) position. This is due, as Sachs has shewn, 

 to the fact that growth has recommenced in the nodes near the 

 base of the haulm, and that it has taken place in such a way 

 that the elongation of the lower surface of the nodes is greater 

 than that of the upper surface. Again, injury to an organ may 

 induce the growth of the cells in the neighbourhood of the 

 seat of injury. Thus, cuts in stems, etc., are healed by the 

 formation of a tissue termed callus, which is formed largely 

 by the renewed growth of cells which had ceased to grow: 

 further, injuries inflicted by insects often give rise to active 

 local growth, of which the formation of galls is a conspicuous 

 example, and this is true also in many cases when organs are 

 attacked by parasitic Fungi. The most remarkable instances 

 of an induction of growth are, however, to be found in con- 

 nexion with reproduction. The egg or oosphere of plants is 

 a cell which has ceased to grow, and if it remain unfertilised 

 it will perish. But when it has been fertilised it grows ac- 

 tively and gives rise to the embryo. In very many cases the 

 stimulating effect of fertilisation is not confined to the egg, 

 but also induces growth in related organs : it is in conse- 

 quence of such an induction of growth that the fruit of 

 Phanerogams, for instance, is formed. 



