12 INTRODUCTION. 



species form an arch of more or less height and extent, but 

 when the shoot again arrives at the ground (if early enough 

 in the season) it is prostrate for some distance, and in the 

 autumn again rises at the end into a very low and small 

 arch so as to present its point directly toward the earth, 

 which it penetrates slightly and takes root. It is convenient 

 to divide these plants into such as are (iii) arcuate, that is 

 form a large and lofty arch the end of which often does not 

 reach the earth until late in the autumn, when its point 

 immediately pushes itself into the ground and takes root; 

 and the (iv) arcuate-prostrate, whose stems, when unsupported, 

 form a very slight and inconspicuous arch, but lie, through- 

 out the greater part of their length, quite close to the 

 gi'ound, often following all the slight inequalities of its 

 surface. The observation of these differences is rendered 

 difficult by the stems being supported by bushes or even by 

 other parts of the plant itself and not reaching the ground, 

 as they would have done if without support. We often see 

 M. discolor, which is an arcuate-prostrate plant, rising out of 

 the tops of lofty hedges, and sometimes rendered unable to 

 reach the ground before its growth is stopped by the winter. 

 "When thus circumstanced it lies upon the top of ■ the 

 hedge in precisely the same manner as it would have done 

 upon the ground if not artificially raised. When so pre- 

 vented from taking its more natural position it frequently 

 forms a knot similar to that of the erect-arcuate plants, 

 and tries by throwing out a slender autumnal shoot to 

 arrive at the earth; or extends its growth from the same 

 point during the succeeding summer, frequently, if the 

 thicket is dense, with a like failure : but where such 

 supports are wanting, its stem will be found to form an 

 arch of only a few inches in height, after which it extends 



