196 LECTURES ON BIOLOGY 



influence of environment is so distinct as in Polygonum amphi- 

 bium, a relative of the well-known knot-grass, which was recently 

 made the subject of important experiments by Massart. This 

 one instance alone would suffice to refute the doctrine of the 

 constancy of organic species and to show that organisms remain 

 alike only under like conditions and change with a change in 

 circumstances. This little plant is exceedingly modest in its 

 demands; it flourishes equally well on marshy land and in the 

 water, and is even found on the hot sand of dunes. But it 

 changes its appearance according to its habitation. At a first 

 glance we are only able to distinguish between the land-form and 

 the water-form, and between this and the dune-form. On the tend 

 Polygonum amphibium is distinguished by an erect stem covered 

 with knots. The small lancet-shaped leaves are, particularly on 

 the under side, furnished with cilia and attached to the stem by 

 very short foot-stalks. In the water-form, however, the stems are 

 much longer, float on the water surface, and grow horizontally. 

 The number of the leaves is increased, they have lost their cilia, 

 changed their form, developed longer foot-stalks, and formed in 

 addition below each knot large rudimentary roots. The difference 

 does not only extend to external appearance but is seen also 

 in the internal structure, for while the stem of the land-form 

 is solid that of the water-form is hollow. In the dune-form, 

 finally, the branches crawl on the sand, the knots are greatly 

 enlarged, the foot-stalks short, the leaves small, thickly covered 

 with cilia, and of a viscid nature. As Massart showed, all 

 these radical differences are the direct result of the influence 

 of environment, for if we lead the branches of the land-form 

 into water the newly-formed leaves and stems assume at once the 

 shape of the typical water-form ; if we transplant the water- 

 form on wet land the stems become erect and form ciliated leaves 

 with short foot-stalks. Finally, if we place the dune-form into 

 water all existing leaves rapidly perish, the stems become longer, 

 and the new leaves are the typical long-stalked leaves of the 

 water-form. 



Let us now consider for a moment the effects of chemical 

 stimuli. It is generally known that the salts and other sub- 

 stances contained in the soil are able to affect the growth of the 

 plants and the production of fruits favourably or unfavourably- 



