72 READINGS IN BIOLOGICAL SCIENCE 



passing to a region of transportation and storage. Here a duct from another 

 needle comes alongside ours; our tunnel will soon be one of a bundle of 

 tunnels. Gradually, with the utmost nicety, the passages of the five needles 

 are merged into one trunk system. We arrive at the stem. 



Here is the junction point for half a dozen bunches of needles. The 

 traffic is as congested and complicated as in a Chicago freight-yard. A 

 hardened botanist may feel assured that all the transfer of water and starch 

 and solids and proteins is managed automatically. Perhaps it is. But if we 

 should try to map the maze of tunnels and the control-points of dis- 

 tribution, we should find that human wit was only dodging the issue when 

 it uses the word "automatic." It would seem just as wise to call the day's 

 work of Chicago automatic. Consider one slight sample — the disposal of 

 some particles of the nitrogen which has been transported all the way from 

 a remote tip of a root. It is very valuable; more of it is required for some 

 needles than for others, and all the needles desire it. Nitrogen has no in- 

 telligence for dividing itself into five unequal portions and going where it 

 is needed. A nitrogen compound is very sluggish and must be handled 

 and shipped by some sort of supervision. What forces apportion it in this 

 labyrinth of busy highways? I am not hinting that there must be intelligence 

 at work; indeed the required skill seems of a higher order than intelligence. 

 I am only protesting that when we say "automatically" we say nothing. 

 The imagination of man has never conceived any term for describing the 

 way in which consignments of sulphur and phosphorus are handled in the 

 node of a twig. 



We return down the twig in a sugar stream, and continue on beyond to 

 the branch from which it grows, a hundred-mile trip. You can read of the 

 layers of which the twig is composed — the bark and cork, the cambium and 

 wood and pith — in any text book. You might while away the hours by 

 speculating on how a twig can increase its diameter each season. Three 

 years hence it will be twice as large; its bark must reach twice as far; yet 

 every minute of the time the bark must remain water-tight. For deadly ene- 

 mies of the tree are always drifting through the air and will invade it if the 

 least spot is left unguarded. Spores of fungi much smaller than the cells 

 would enter the wood and feed upon its stores of food as certainly as 

 bacteria always enter any broken skin of an apple and cause decay. So 

 the bark must always be sealed. How does it increase its circumference 

 without permitting any microscopical gap at any moment? 



We enter the branch at a node where three other twigs converge with 

 ours. We are to travel the length of it to where it joins a larger branch — 

 somewhat more than the distance betu^een New York and Chicago. Here 

 the branch is only seven miles in diameter, and at its base is only ten miles 

 in diameter. Somehow this seems impossible architecture. We can conceive 

 a cable of those dimensions, but we can not conceive how the cable could 

 be transformed into a stiff, staunch girder, bent through three ungainly 



