516 EFFECTS OF GROWING CELLS ON ENVIRONMENT. 



and press asunder small stones and clumps of earth. In many plants the growing! 

 points of the runners are covered with hard scales, which produce exactly the same 

 effect as the points of an auger. This applies especially to several grasses {e.g. . 

 Galamagrostis, Lasiagrostis, and Agropyrum). The runners of the common creep- : 

 ing Couch-grass (Agropyrum repens) bore through the roots of trees, and not only 

 through old and rotten but also through young vigorous specimens. The runners 

 of the Couch-grass are often found penetrating through the centre of potato- 

 tubers, and it has been confirmed experimentally that these runners in their growth 

 are capable of even boring through discs of tin-foil. Very instructive also is the 

 penetration of old tree-trunks by the stems of various small shrubs and shrubby 

 trees whose growing points are comparatively delicate and soft in texture, and are 

 not beset, like those of the Couch-grass, with hard pointed scales. Almost every- 

 where in our mountain regions are to be seen, in places where not very long before 

 a forest has been cleared, dead stumps of fir-trees, rising perhaps a metre above 

 the ground, and overgrown with cranberry and bilberry bushes. The surface : 

 where the saw has cut through the huge trunk is partly overgrown with the same 

 plants as those growing in the soil round about, and it has a very peculiar appear- 

 ance when on these decayed stumps, as if on the platform of the base of a pillar, 

 small colonies of cranberry bushes are seen to flourish luxuriantly — a story 

 higher than on the surrounding ground. Without closer investigation anyone 

 would think that these bushes had germinated from seeds which had previously 

 fallen into the cracks of the stem section, and it is not a little surprising therefore, 

 on splitting such old tree-stumps, to find that this is not the case, but that rather 

 the cranberry bushes of the surrounding forest-ground have sent out their runners 

 into the lower portion of the tree-trunk, and that these have then grown up through 

 the rotten wood of the stump — especially through the decayed part between the wood 

 and bark — until they have again reached the daylight above on the exposed section, 

 showing at any rate that they must have exerted a very considerable pressure on 

 their surroundings. The thin stems, also, of plants growing on boulders have 

 frequently to make a new pathway for themselves when their habitat has been 

 covered by a torrent with sand and stones a span high, and thus have to push out 

 of the way obstacles of comparatively large dimensions. On a forest soil covered 

 with sand and boulders I saw indeed how the delicate thread-like stem of a winter- 

 green (Pyrola secunda) had grown up more than 60 cm., and in doing so had pushed 

 on one side stones of a gramme weight. If peas, beans, and other large seeds are 

 buried in the earth and allowed to germinate, it may be seen how by the growth 

 of the seedling small clods of earth and stones are raised, and the earth in which 

 pine-seeds, oats, and beech-nuts have been embedded, looks when the seeds are 

 germinating as if it had been rummaged and thrown up by mice. A fine example 

 of external work done by growing stems must yet be instanced in the growth in 

 height of the forest-trees which we have daily before our eyes, but only too easily 

 overlook on account of its commonness. A young beech trunk 50 cm. thick will 

 raise each year a crown which has a weight of two thousand kilogrammes 



