EXTERNAL CAUSES OF GROWTH AND FORMATION. II 315 



portion to increase in weight. [Keller (1904) has recently shown that in the 

 peduncles of ripening fruits there is an increase in the tendency to rupture, but 

 he (as well as Wiedersheim and Ball) has shown that this is not due to in- 

 creased weight.] Hence we obtain morphogenetic responses as the result of 

 the action of mechanical factors, responses which are frequently observed in other 

 cases. Thus, by bending of the main root the development of lateral roots 

 from the concave side may be prevented, they then arise only on the convex or 

 stretched side (Fig. 92, Noll, 1900). Very often we may observe results of 

 pressure manifesting themselves with different intensity in adjacent parts 

 of the same organ. Such 'contact sensitivity' is illustrated by roothairs, 

 which exhibit a diminution of growth when in contact with the soil particles, 

 accommodating themselves in the most complete way to the inequalities of 

 surface. Contact responses of a special kind are exhibited by the tendrils ot 

 species of Ampelopsis which form holdfasts by pressing their apices in close 

 contact with some solid body (Lenger- 

 KEN, 1885). Lastly Mucor stolonifer pro- 

 duces stola whose apices, when they come 

 in contact with the substratum, attach 

 themselves to it by means of rhizoids, 

 the plant thereafter proceeding to form 

 sporangiophores (Wortmann, 1881). 



The examples cited must suffice to 

 illustrate the influence of pressure and 

 tension on growth, and we must now 

 turn to the consideration of certain 

 other external influences on growth 

 which act sometimes chemically, some- 

 times mechanically. Since all growth 

 is dependent on the supply of nutri- 

 tive substances, it follows that the 

 essential nutrients we have already re- 

 ferred to may be considered as con- 

 ditions of growth, and each of these 

 must be present in a certain minimum 

 quantity before growth takes place. 

 An excess of the other nutrients does not 

 compensate the plant for the deficiency 

 in one. Under such starvation con- 

 ditions growth results for the most part 

 in a diminution in the size of the whole 

 plant. Heinricher (1896) has described plants of Sinapis nigra, which when 

 sown closely on bad soil reached a height of only 18 mm., but which formed 

 both flower and fruit. Similar dwarf plants were obtained by LOpke (1888) in 

 cultures without potassium ; a similar case is shown in Fig. 93. The reduced size, 

 which many plants exhibit in the absence of nutritive materials, may be con- 

 sidered as a phenomenon of adaptation, since the organism is able in this way to 

 complete its life-cycle, while, by employing the nutrients available for the forma- 

 tion of organs of normal size, only a single leaf perhaps could be formed, and 

 the development would then come to an end. The dwarfing of all the organs, 

 i. e. harmonic dwarfing, as we may quite correctly term it, is, however, not the 

 only response given by the plant to absence of nutrients ; in some cases inhar- 

 monic growth also results. We find, for example, that when nitrogen is absent 

 roots, roothairs, and internodes undergo excessive elongation (Noll, 1901 ; 

 Benecke, 1903). We may term this ' etiolation ', and biologically it is obviously 

 closely related to etiolation due to absence of light. As the individual nutrients 



&' 



Fig. 92. Young lupin with a cur\'ed principal root. 

 The lateral roots are developed exclusively on the 

 convex sides. After Noll. From the Bonn Text- 

 book. 



