404 



MEMOIRS OF THE AMERICAN ACADEMY. 



♦ 



youn 



Next to the advantage of being near a market, or a source of 



pplies, is the advantage of being in the line of traffic 



Th 



indeed 



part what it is to be a market, rather than a mine. A leaf is not 



nly a 



productive or industrial centre, but a commercial 



It effects exchanges, both 



giving and receiving supplies. When mature, or fully established in this capacity, 

 it draws from the roots its raw material of water and mineral salts, and from 



the 



air 



its 



m o re 



costly 



material, and in exchange sends forth into the 



commerce of the stem its \ 

 sunbeam, its soluble colloids 



derfully 



fab 



of atoms 



great 

 on the 



No 



although sap may flow 



th 



rly 



qual 



facility* in all directions in the stem, it probably does flow with greatest rapidity 



in the direct lines of the forces that 



pel 



the lines of osmotic force 



Sap 



g which is 



flows most freely from that side of a perforated tree in the Sprii 

 immediately below the largest branch. This shows that even in the least active 

 condition of the circulation, when the trunk is surcharged with sap, the forces 



of circulation are not 

 less so when definite 



buds 



simply diffusive or hydrostatic ; and they must be much 

 outlets of this supply become established in the growing 



d leaves of the 



sprm 



The character of the circulation is 



pally determined by the hydraulic action of osmotic forces. Water 



pn 



with ( 

 but it 



qual facility in any part of a 



ay flow 



bed, and across as well as along 



tually does flow fastest along the middle 



Th 



ferent need 

 competitors 



growing leaf has dif- 



from those of the mature one; hence they are not rivals, or 

 the market, but buyer and seller, or borrower 



and lender. 



The 



mature leaf needs from the stem water and mineral salts; the growing leaf 



ds the organic materials of new tissues. The mature leaf helps to 



the latter by concentrating 



prepare 



thdrawing the 



tribution of organic material in return. 



and adding its own con- 



But while 



in this way 



ding its 



y 



fellow 



aided in return, or its efficiency is increased, by the increased 



circulation produced through the forces of movement above it. 



In 



glut in the market we have an active 



of a 



exchang 



There is, undoubtedly, a 



tendency in these physiological causes, however feeble, to that vertical allig 



ment of not very distant leaves, which the 



ycl 



character of the 



ipiral 



arrangements exhibits, and most markedly in the J or alternate system 



We h 



thus 



features in the particular forms of th 



gned more or less probable utilities to two prominent 



leaves; their distrib 



spir 



and 



ticil 



gements of 



and cyclic characters. We now come to a much more 



in general 



obscure problem, which connects the verticil and spiral arrangements 



