37 Ü •; 76 



The biological advantages of the underground runners consist in furthering the 

 migration of the individual to richer soil, occupation of new space with more food, increas- 

 ing the number of aerial shoots, and reproduction of the species, and thus also the prospect 

 of victorj' in the struggle against coniiieting forms. Many species having their underground 

 runners higly ramified become extremely social (e.g. Urtica diacca, Asperula odorala, Stellaria 

 ncmoriim and holoslca, Ægopodiam jmdaiiraria, Mercurialis peicnnis. Pliranmiles, Tijpha, 

 Psamma arenaria, Equiseiam, etc. This is greatly facilitated when several generations of 

 shoots can arise during the same growth period (ex. gr. Phrat/miles communis; see Warming 

 1884, lig. 10), and additional buds be developed. Wallace is of opinion that there are more 

 social plants in fairly temperate climates than in the tropics iWorld of Life, 1910: 98). This 

 agrees with my observations. The other underground i)lagiotropic shoot-types can also pro- 

 duce a rich social growth. 



The plagiotropic direction of tlic runners in the earth is naturally in accordance 

 with their biological function. It must undoubtedly be dependent on the whole upon the 

 force of gravity, as first pointed out by Elfving. In Tussilago farfarus, growing on a steep 

 slope, I found the direction of growth of the runners to be parallel with the slant of the 

 surface (fig. 43). It seems hardly possible that it should in this case be dependent either 

 ui)on gravity — see direction of the roots — or upon light, but rather upon conditions in 

 the soil itself, such as moisture, or composition and quantity of the air etc. 



The curving section of the runners. Sooner or later, the runners change their di- 

 rection, and turn olf upward in an orthotropic aerial shoot. In some species, the curving sec- 

 tion of the stem is not jiarticularly pronounced (fig. 1, 11, 15). In many forms, however, this 

 part has far shorter internodes than the remainder of the runner, and is often, moreover, 

 thicker, besides being rich in roots (see ]). 354 for reference to figures). F"rom this portion 

 of the shoot, we often find many lateral shoots proceeding, and a mesocorme is formed; 

 in Convallaria majalis, for instance, it may continue to increase in height for several years. 

 A contributory cause of this thickening of runners of the jiart in question is perha|)s that 

 the fiow of nourishment to the aerial shoot is here impeded, whereby an accumulation of 

 nutritive matter is formed. 



From here we are led to those runners wliich terminate in a food-accumulating organ, 

 such as tubers or bulbs, the formation of which as a rule involves the early death of the 

 thin first part of the shoot (figs. 33—42). 



The longevity of the runners is, it should be noted, highly variable, de])ending partly 

 upon the nature of the habitat, partly ui)on the biological importance of the runners them- 

 selves. The drier their habitat, the longer they will live, and cases are known where both 

 the primary root and the runners with their secondary rootlets become more or less lig- 

 nified (species of Hypericum, Galium, Papilionacece etc.). 



On moist ground, or in water, the runners decay more rapidly, esjjecially when the 

 apex is transformed into a feeding organ e.g. Mentha, Lijcopus, Lijsimachia iniltiaris, C.ircœa, 

 see fig. 7, 35, and the potato type mentioned on |), 337 — 344). Many species become i)seu(loannual: 

 at the end of the growth jieriod, only rejuvenating buds arc left in the earth (see the species 

 mentioned on p. 355—556: Chrysosplenium, (Arcwa, especially ('.. ulpina, etc.). 



There are in our northern flora many other species, belonging to other types of growth 

 forms, which also are pseudoannual (e.g. Adoxa moschalellina. Arum maculatum. Ophrt/deæ, 

 etc., see p. 356). 



A peculiarity of many pseudoannual species, esi)ecially those furnished with food 

 accumulating organs, is that the aerial shoots are unbranched, or only slightly branched 

 (e. g. Ophrijdeae. Epilolnum monlanum). This is perhaps in correlation with the fact that the 

 nourishment has more particularly to be carried to the new rejuvenating buds. 



Formation of the runners. Avery striking feature is the strength with which 

 branching takes place from the base of the primary shoot, branches being formed from the 

 axils of the cotyledons, sometimes from there alone, sometimes also from the axils of leaves 

 immediately above. Then also there are often accessory shoots serially under the main 



