664 PROCUMBENT AND FLOATING STEMS. 



on them. The long bare internodes are always thin and thread-like, and perish 

 in the course of a year. One portion of the buds developing at the nodes of the 

 runner forms short axes; another part may form even in the first year long axes 

 which again assume the form of a runner. Since each plant sends out simul- 

 taneously several runners extended on the ground on every side, it comes to pass 

 that in a very short time considerable areas are spun over in all directions with 

 filamentous runnei-s and a host of new plants produced. Well-known examples 

 of this form of procumbent stem are furnished by the strawberries (e.g. Fragar-ia 

 vesca, grandiflora, Indica), several cinquefoils {e.g. Potentilla reptans and 

 Anserina), the Creeping Avens {Geuvi reptans), the Stone Blackberry {Rubus 

 saxatilis), the Ground Ivy (Glechoma hederacea), and the Japanese Saxifrage 

 {Saxifraga sarmentosa). A very peculiar appearance is presented by Androsace 

 sai'Tiienfosa, which grows in the Himalayas. All its leaves are crowded together 

 into a beautiful rosette on an erect short axis. From the axils of several of these 

 rosette-leaves, long, thin runners, red in colour, radiate out during the summer; 

 they extend themselves on the rocky soil, and each runner forms at its end only a 

 single rooting bud or rosette. The red filaments perish in the second j'ear, but 

 by this time five or six freshly-rooted rosettes may generally be seen standing ia 

 a circle round the older one. 



In a third group of plants the whole procumbent foliage-stem with all its 

 branches dies ofi" ever^^ year at the close of the vegetative period. The plants 

 belonging to this group are either annuals and maintain themselves only by 

 seeds, or they possess perennial subterranean scaly stems, in which case each 

 year new leafy stems arise. The foliage-stem of these plants is said to be 

 " prostrate " (stirps humifusa). The following may serve as examples of such 

 annual, prostrate shoots:- — -The Caltrops (Tribulus), the Strapwort {Corrigiola), 

 Illecehruvn, the Pimpernel (Anagallis), the Ivy-leaved Speedwell ( Veronica hederi- 

 folia), the Portulaca {Portidaca oleracea), and numerous species of Polj'gonum, 

 trefoil, and medick (Polygonum, Trifolium, Medicago); as examples of perennial 

 prostrate plants — the Bird's - foot Trefoil (Lotus cornicxdatus), the variegated 

 Coronilla (Coronilla varia), and several caryophyllaceous plants (e.g. Saponaria 

 ocymoides, Telephium Imperati). 



When the leafy shoot lies on the soil, it can easilj^ dispense with the develop- 

 ment of those cells which would otherwise be required to give to its stem strength 

 for support and resistance to bending. Thus plants with pi'ocumbent stems 

 have an advantage in this respect over such as stand erect, in that they can 

 economize so much building material. On the other hand, however, the pro- 

 cumbent form has the disadvantage of being able to expose to the light relatively 

 little green tissue; only those of its leaves can be well illumined which are 

 arranged like a mosaic in a plane parallel to the substratum. The development 

 of a second such layer of leaves higlier up would be a decided disadvantage, for 

 it would cause the lower stratum of foliage-leaves to turn yellow and pine away. 

 Consequently, any upward extension of the green tissues in procumbent shoots is 



