800 THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



which become independent by the gradual withering and decomposition of the 

 connecting pieces of thread. They form spherical spore-capsules, and with the 

 scattering of the spores they wither and die. These plants are only a few milli- 

 metres high, but they are clustered together in such thousands that they form a 

 velvety carpet over the soil, their emerald-green colour being the more striking as 

 the last remains of the neighbouring vegetation have assumed the dull hues of decay. 

 The Luminous Moss (Schistostega osmundacea) growing in the holes and clefts of 

 slate mountains (already described in vol. i.), the protonema of which is depicted in 

 Plate I., also forms loose colonies of separate Moss-plants from the green threads 

 which creep over the clayey soil in the hollows. These plants die off after they 

 have ripened their fruits. Of course the development is in this case not so rapid 

 and does not occur in the late autumn as in Pottia intermedia. 



A peculiar formation of offshoots may be noticed in epiphytes which climb over 

 the bark of old trees and possess only short ribbon-like roots adhering to the damp 

 bark, but none which grow down into the ground. Their stems and leaves invest 

 the substratum like a carpet, as, for example, in several tropical Aroids of the genus 

 Pothos, and in Marcgravia. The growing stem forks, and later on by the dying 

 away of older portions behind the fork the two branches are separated and isolated. 

 Each in its further growth may go a different way, one climbing up this and the 

 other up that branch of the tree-trunk which serves as support; and, since this 

 process is repeated, several independent plants of Marcgravia and Pothos may be 

 found on the crown of the tree, all of which are to be regarded as natural offshoots. 

 The same thing occurs in numerous Ferns, which grow on the bark of trees and in 

 the humus-filled clefts of rocks, and in all those plants whose creeping aerial stems 

 grow and branch at one end while they die off to a corresponding extent behind, as 

 in many creeping species of clover, for example. As the annual increase in the stem 

 of these plants is but small, the separated individuals move very slowly from one 

 another, and several years elapse before the offshoots have formed a group which 

 extends over an area of a square half-metre. 



The result is obtained comparatively much quicker when the offshoots are formed 

 by runners and shoots. In one section of these plants, of which the Saxifraga 

 flagellaris (fig. 446), a plant widely spread through the Arctic region and in the 

 high mountain districts of the Himalayas, Altai, and Caucasus, may be taken as a 

 type, only a single bud is developed at the end of a slender thread-like shoot. This 

 takes root where it touches the ground, and grows up into a rosette. Not until the 

 nourishment of the rosette by the rootlets which have been sent into the ground is 

 assured does the long thread, terminated by the bud, die off, the connection with 

 the mother-plant being thus severed, whilst the rosette now forms an independent 

 plant. Since the shoots are usually numerous and radiate outwards the mother- 

 plant in course of time becomes surrounded with an actual garland of rosette-shaped 

 offshoots, and in a few years a fairly large area is covered with hundreds of larger 

 and smaller rosettes, which, however, no longer show the circular arrangement, because 

 the shoots of neighbouring rosettes often cross, and consequently the circles intersect. 



