372 78 



low down or two high up, must curve up or down accordingly. Experiments have been 

 made and illustrations given by Rimbach and Raunkiær (1907, fig. 46, 47, 48); see also P.E.Müller 

 1894. As to the factors which start these growth movements, we know nothing certain. 



When an earth-stem lies too high, it can be sunk deeper down by various ways. 

 Passive descent is elTected by the de])0sit of earth or other matter on the surface of the 

 ground. P. E. Müller has shown the importance of the activity of earthworms in lowering 

 the level of earth stems. 



Active descent is accomplished in at least two ways. One is the contraction of the 

 roots, already observed by Tittmann so far back as 1819, and later by Irmisch, Hildebrand and 

 others, especially by Rimbach (in a series of works from 1893 to 1902). This appears to take 

 place more particularly in a number of species with niesocornie and strong primary root, 

 as also in many bulbous and tuberous plants. In species with underground runners, this 

 form of movement will be less common, save in cases where the species retains its primary 

 root for a long time, as in Ægopodium (fig. 29, p. 333), presumably also in Saponaria offici- 

 nalis (p. 313, fig. 11) and perhaps Honckcni/a (p. 313, fig. 12) and likewise, for instance. Cam- 

 panula rapuncoloides etc. 



The other method is that of curvature in ])lagiotropically growing stems (underground 

 runners, rhizomes, rhizodes) witii roots too weak and non contractile (e. g. according to 

 Rimbach, in Majaiillnmiim hifoliiim, Polycjuniitum miilli/lorum, Valeriana officinalis, Dcnlaria 

 hiilbifcra (fig. 2, Circwa Inleliana (fig. 6, 7), Adoxa moschcllatinu etc.). Here the growing 

 I)oinls of the earth-shoots alter their growth direction, tending downward when they have 

 reached too high a level, and up when they find themselves too far down. The rhizome of 

 Dcnlaria bulbifera bends in a series of obtuse angles when working down year by year 

 deeper into the ground (Warming 1876); the roots are too few and too thin to be capaljle of 

 drawing down. See also Rimbach. 



We know nothing definite as to how the plant iiciceives that it is not at a ])ro|)er depth, 

 or what forces aid it in seeking the same. Rimbach (1898 p. 203) states that he has never seen 

 a rhizome alter its direction before it had sent up an organ above ground to the light; this 

 organ, which in the young Denlaria earth-stems is a leaf, in the older ones a flowering aerial 

 shoot, should thus in some way or other inform the growth point of the depth at which it 

 lies. Rimbach opines that it must be the "internal ecomomy" of tiie jilant which occasions 

 the alteration in growth, and (Czapek is said to hold the same view. Raunki.kr also considers 

 that the plant is able by means of its aerial shoot to measure the distance from the rhizome 

 to the point where the erect aerial shoot touches the surface of the earth, and he has attempted 

 to support this view by an experiment. He placed a zinc cylinder over the spot beneath 

 which the rhizome of a Polijgonalum mulliflonim was laid rather high up. It was then 

 seen, that the darkened layer of air acted just as the earth ordinarily does, and the riiizomes 

 whose aerial shoots only reached the light after having grown tlirough the darkened hiyer of 

 air, were much bent upward. The aerial siioot should thus be able to perceive an excitation, 

 and conduct it through that part of the shoot which is in darkened air down to the rhizome, 

 where the reaction then first takes place. Whether this explanation is correct 1 do not know; 

 the earth under the zinc cylinder would of course undergo a change in respect of moisture, 

 temperature, and air content, which points should also be taken into consideration. 



It seems to me, that the most natural explanation would be that factors in the soil itself 

 (moisture, quantity and character of the air — oxygen content in jjarticular — etc.) must de- 

 termine the de|)th to which the earth stem rises or descends. When, on the other hand, an 

 underground runner turns upward in the curving section in order to grow out above the 

 earth in an aerial shoot, then internal factors must be considered to be at work. 



Adaptation to habitat. The question naturally arises whether underground runners 

 can be regarded as ecologically adapted to their habitat. A habitat is, however, a so complex 

 factor in itself, that the easiest method to which we naturally turn is a statistical reckoning of 

 the number of species with underground runners which are attached to the most widely ditl'e- 

 rent types of habitat found within a given area. In the case of Denmark, such a method 



