S32 THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



which they have been shifted under the ground. Some soil containing bulbs of 

 Tulipa sylvestris was once put in a garden in Vienna in the middle of a grass plot 

 shaded by Maple-trees. As the grass was mowed every year before the flowers 

 opened there was no formation of seed, and the Tulips could only multiply by 

 offshoots. After about 20 years the lawn was covered with Tuhp-leaves, which 

 arose from subterranean bulbs occupying an area 10 paces in diameter. Thus in 

 the time mentioned the bulbs had spread for about 5 paces in all directions, in 

 consequence of the pull of the contracting roots. It is more than probable that the 

 offshoots of many perennial plants with erect stem and napiform or tuberous roots, 

 e.g. the blue-flowered species of the Monkshood (Aconitum Napellus, A. JSfeubergense, 

 A. variegatum) undergo a change of position by the pull of their horizontal root- 

 flbres; and that the clustered arrangement of these plants is the result of the root-pull. 



A review of the very varied modes of origin and distribution of offshoots leads 

 to the conclusion that they may be formed on all parts of the plant, that the form 

 of the offshoot is constant for each species, or, in other words, that the form of the 

 individual parts of the offshoot in succeeding generations is repeated as exactly as 

 the flowers and fruit, but that one and the same species may frequently form two 

 or even three kinds of offshoots. The Fungus Glaviceps purpurea develops 

 spores which are distributed by honey-sucking insects, also the sclerotia known as 

 " ergot ", which are scattered from the dry spikes by the swaying movement of the 

 stem, and thirdly, filamentous spores, which are extruded from asci, and distributed 

 by wind. The Liverwort Blasia pusilla, develops thallidia in special flask- 

 shaped receptacles on the surface of the thallus, and spores in the sporogonia. The 

 form of the offshoot is always adapted to the season and to the distributive agents 

 available where they are formed. In one case it is more suitable that the offshoots 

 should be distributed slowly, and step by step, in another quickly and by bounds. 

 In the spring it may be more advantageous if they are distributed by wind, by 

 animals in the summer, and by self -scattering mechanisms in the autumn. Steppe- 

 plants must develop different offshoots from those formed by plants living on the 

 damp, shady floor of the forest. It is just as obvious that offshoots, which creep 

 along, above, or under the ground without leaving the soil, must be equipped quite 

 differently from those which are detached from their place of origin, and either roll 

 along or are carried by wind, or have to travel long distances as the appendages 

 of wandering animals. In the former, it is all-important that they should be able 

 to overcome possible obstacles in the soil; in the latter, that they should not perish 

 during their journey for lack of food and water. When separated from the soil 

 they are greatly exposed to the danger of drying up, and even when they have 

 settled somewhere, the supply of water they require for the formation of organs of 

 attachment and absorption is by no means assured. Settlers of this kind must 

 either be so .organized that they can sustain a long-continued drought without 

 injury, like the offshoots of the Mosses and the soredia of Lichens, or they must 

 themselves bring with them the necessary water supply, and care must be taken 



