768 EEMARKABLE PROPERTIES OF ROOTS. 



settle in the clefts and crevices of perpendicular rock faces, a phenomenon is 

 produced by this down-drawing which surprises anyone noticing it for the first 

 time, appearing at first quite inexplicable. The thick stems of these primulas 

 (e.g. Primula Auricula, Clusiana, hirsuta) are terminated by a rosette of foliage- 

 leaves; these turn yellow, and dry up in the autumn, and a new rosette is laid 

 down in the axil of one of them for the next year. Although the leaves of the 

 rosettes stand close above one another, the portion of the stem clothed by them 

 is quite a centimetre long, and the annual increase undergone by the stem which 

 grows towards the light is also a centimetre. The increase during ten years would 

 amount to 10 cm., and it would be expected that the rosette of the tenth year 

 would be about 10 cm. above the level where stood the first year's rosette. But, 

 .strange to say, the rosettes of all the succeeding years always remain at the same 

 place, that is, they always cling to the rocky edges of the crevice or cleft in which 

 the stock is rooted. The explanation of the phenomenon is that the roots springing 

 from the I'osette-bearing stem draw it down every year about a centimetre into 

 the soil or crevice filled with humus. But naturally this can only occur if the 

 lower end of the stem annually dies ofT and decays to a corresponding extent, 

 and this is wliat actually happens. In rocky clefts which are not well adapted 

 to this process the primulas grow badly, and their stems project above the edges 

 of the crevice; ultimately the entire plant falls into a slow decline and no longer 

 blossoms, but perishes in a few years. The knowledge of their peculiar mode 

 of growth is therefore of some importance in the cultivation of these primulas, 

 since care can be taken to plant them so that the stems can be annually drawn 

 i certain amount into the soil by the roots. It is of course needless to mention 

 that many other plants beside primulas, rooted in crevices of rock, behave in the 

 ^ame way, e.g. Phyteuma comosum, Gentiana Clusiana, Campanula Zoisii, 

 Paederota Ageria. 



The ends of branches of many species of bramble are drawn under the ground 

 in a very peculiar way. One of these species, Rubus hifroyis, is represented in 

 fig. 188, where the roots and the ends of the branches drawn under the soil 

 by them are rendered evident, the earth in the foreground being removed as if 

 dug away by a spade. Rubus bifrons develops strong five-ridged shoots beset 

 with reversed prickles; they at first grow directly upwards, but towards autumn 

 hang in broad curves, so that their tips approach the ground. Before they have 

 reached the soil, however, small scale-like protuberances, looking like stunted 

 leaves, are to be noticed arising near the tip; these ai-e the commencements of 

 roots. When the apex of the branch at length trails on the ground the protuber- 

 ances, now in contact with the soil, elongate into roots which peneti'ate the ground. 

 They lengthen very rapidly, sending out numerous lateral roots, and in a short time 

 an extensive subterranean root-system is the result. But the apex of the branch 

 which serves as a starting-point for these roots, and which is now considerably 

 thickened, has also come under the ground. It has been drawn down by the 

 roots, and is now embedded in the soil. In the following spring, sometimes even 



