798 THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



The rapidity of the extension and the dimension of the colonies which are pro- 

 duced from rhizomes and runners depends upon the length of the annual shoot, and 

 upon whether or not the soil is favourable to the spreading of the offshoots, just as 

 it does in the linear or clustered colonies arising from underground roots and tubers. 

 In wood-clearings and on the banks of rivers many of these plants develop in a 

 surprisingly short time, as, for example, Calamagrostis Epigeios, Epilobium angusti- 

 folium, the North American Golden Rod and Rudbeckias (Solidago Canadensis 

 and Rudbeckia laciniata), and these also have the property of suppressing and 

 destroying all other vegetation in places where they have taken possession. This 

 fact is turned to practical account by farmers who use certain Grasses which form 

 linear and clustered offshoots to bind together loose soil, especially river sand. But 

 there are also plants in this category which are veritable plagues to the farmer, the 

 establishment and propagation of which he opposes by every means in his power. 

 Examples of these dreaded plants, which, when they establish themselves in the 

 fields and garden-beds, hinder the development of other plants, are furnished by the 

 Gout-weed {^gopodium Podagraria), the Stinging Nettle (Urtica dioica), and 

 the Fuller's Thistle (Cirsium arvense). Wherever these have settled on cultivated 

 gi-ound and penetrated the soil with their ofishoots there is nothing for it but to dig 

 up the whole ground and to carefully remove all the shoots. Unfortunately even 

 this laborious task is not always rewarded with the desired result, for in spite of the 

 utmost care it may happen that small fragments remain, and these form the nucleus 

 of a new colony of weeds. In a short time a new group appears above the soil 

 which has been cleared with so much care, and a fresh digging and still more careful 

 clearing of the ground is necessary. These clustered colonies have a characteristic 

 appearance when foliage-leaves spring from their underground stem-structures, the 

 large blades being borne on almost equally long erect stalks, as is the case, for 

 example, in the Butterbur (Petasites officinalis) and numerous tropical Aroids. 

 Wide tracts are then to be seen covered exclusively with their large luxuriant foliage- 

 leaves, all other vegetation being suppressed. The formation of offshoots and the 

 production of clustered colonies also occurs to a remarkable extent in the common 

 Reed (Phragmites communis). Once settled on suitable soil it will cover the 

 widest areas in uninterrupted and unhindered march, suppressing and destroying 

 all other plants. On the lower Danube there are many lowlands so thickly set with 

 Reeds that in several hours' journey only a few small inconspicuous plants will 

 be seen beside the Reed haulms. This Reed is also interesting from the fact that its 

 offshoots can arise just as well under water as under the ground, and it may serve, 

 in some respects, as the type of a group of plants which, by reason of their 

 amphibious nature, play an important part in the transformation of submerged into 

 dry land. 



On the other hand, the variety of the protonemal threads, runners, shoots, and 

 creeping stems which spread above-ground from the offshoots of these colonies is 

 almost inexhaustible. And this is readily intelligible. The processes which are 

 connected with their formation are much more varied in plant-members which grow 



