McDonald : The Dog's Mercury. 



265 



A shoot similar to those marked C and C^, Fig. 7, was ob- 

 tained by the writer, which measured over a foot in length 

 from its base to the point where it emerged from the soil ; it had 

 three internodes. 



c 



Fig. 7. — Diagram of underground branching of a mature plant, 

 end of September ; a, decayed base of aerial shoot of last year ; st. unde- 

 cayed underground portion ; b, b,"^ points to which aerial shoots of this 

 year will decay; c, c^, where aerial shoots for next year will emerge, 

 dotted portions represent further growth of the at present horizontally 

 growing shoots c, ; r, roots from nodes; s, a little above surface 

 of soil ; V, vertical shoots that take no part in the spreading process. 



For details of the manner and advantages of the arched 

 method of emergence, particularly in respect to seedlings, the 

 reader is referred to Darwin's book, ' The power of movement 

 in plants.' 



In the Dog's Mercury, the young leaves have each half of 

 the blade rolled to the middle of the upper surface (involute). 

 The rather thick and tough midrib projects and helps to protect 

 the lamina. All the leaves and stipules fit together at this 

 season in such a way that they form a conical surface attached 

 at its narrow end to the stem. They offer little resistance, 

 and the displaced particles of soil slip over without injuring 

 as the cone is drawn upwards. Speaking of the arching of the 

 hypocotyl in respect to seedlings, Darwin says ' that an in- 

 creased force is also gained by the arching, for both legs of the 

 arch increase in length and both have points of resistance, 

 the basal leg meets with resistance by the root at its base, and 

 the apical leg meets with resistance so long as the seed leaves 

 are still in the testa, so that the arch is pushed up with a double 

 force.' The basal leg of an arched shoot of Dog's Mercury 

 meets with resistance, and it is possible also that owing to the 

 rolled leaves growing in length whilst still in the soil, their tips 

 meet with resistance by the soil and if so, to some extent, the 



1908 July I. '-' 



