PROCUMBENT AND FLOATING STEMS. 663 



In addition and in contrast to this first group of plants with procumbent foliage- 

 stems there is a second, characterized by the fact that only the buds arising on the 

 new shoots remain throughout the year, strike root, and grow out into new plants, 

 while the shoots themselves the axes from which the buds have been developed 

 soon perish, thus severing the connection with the parent plant. These shoots are 

 always thin, frequently quite thread-like. Little building-material is wasted upon 

 them, since they are but ephemeral structures. Two distinct types of stem may be 

 distinguished in this second group. These are known as the stolon and runner. 

 By " stolon " (stolo) we understand a procumbent stem which dies off after a year, 

 and is abundantly beset with leaves not very far apart. In the axils of many of 

 these leaves no buds are produced, and often only at the ends of the stolons do buds 

 arise from the axils of very minute leaves; these buds take root. This is especially 

 the case in the arched stolons, as, for example, in the well-known Periwinkle ( Vinca), 

 and the purple Gromwell (Lithospermum purpureo-cceruleum). The shoots arising 

 from old plants of these species form flat arches abundantly beset with pairs of 

 leaves. Their free ends lie on the ground, swell and grow down into some dark chink 

 or into the black humus itself, striking root and thus being drawn still deeper into 

 the ground. The end of the stolon, thus embedded, finds itself next year, so to 

 speak, on its own feet; it grows up into a new plant, while the arched or connect- 

 ing portion dies off sooner or later, and in the following year, or the year after 

 that, vanishes, leaving no trace. The stolons of the Pennywort (Lysimachia 

 Nummularia) are similarly constructed, but in this plant the shoots lie flat on the 

 soil, and the tip does not thicken, nor do the apices avoid the light, or become drawn 

 far into the earth. Rooting buds arise in the axils of small leaves close to the 

 up-bent apex of the stolon, and in the following year become starting-points for 

 new plants. Several species of saxifrage and house-leek (Saxifraga and Semper- 

 vivum), the Common Bugle (Ajuga reptans), some hawkweeds (e.g. Hieracium 

 Pilosella and Auricula), and numerous other plants develop richly-leaved stolons 

 which produce at their free ends short axes which root. The leaves on these short 

 axes are grouped in rosettes; the short axes grow next year into new plants, 

 the intervening stolon perishing. A peculiar modification of this method of growth 

 is found in certain house-leeks (Sempervivum arenarium and Soboliferum). Here, 

 as before, the tip of the thread-like stolon develops a short axis with leaves 

 arranged in rosettes, but as soon as this is fully formed, the stolon withers, the 

 spherical rosette becomes detached from it and rolls down over the steep ground 

 where it had developed. Since these species of house-leek grow as a rule on the 

 narrow ledges of precipitous rock-faces, it happens that the rosette thus detached 

 falls from ledge to ledge, often to a depth of many metres, truly a remarkable method 

 of distribution, which we shall allude to again in the second volume (cf. vol. 



fig. 425). 



= The "runner" (sarmentum) is distinguished from the stolon by the fact that 

 its internodes are much elongated, and that leaves and buds which stnke root 

 and form the starting-points of new plants, are only formed at wide intervals 



