yo READINGS IN BIOLOGICAL SCIENCE 



yield. All this hard-won booty must be transported through the roots, up 

 the trunk, along the branches, into the twigs, by leaf ducts, to the thirsty 

 cells. Water is costly, the plant's greatest treasure. Yet it leaks away from 

 the cells, out through their walls, and escapes from the funnels of the 

 breathing-holes. Why should there be such waste? Simply because the 

 needle is an imperfect apparatus. It works well enough to maintain a hardy 

 tree in soil that is not too poor. But it is far from perfect. Here is a text for 

 a long essay on evolution as a process of adjustment. All the adaptations 

 of plants and animals are of that sort — fairly good, sometimes astonishingly 

 good, but never complete. Evolution has been a series of makeshift con- 

 trivances. 



Still this chamber of gases is by no means so illfitted for its purpose as you 

 might imagine. The cells which open and close the outlet are automatically 

 influenced by the supply of water in the cells; as the supply is depleted, 

 the cells stretch out and make the hole smaller; they will close completely 

 if a hot, dry atmosphere is robbing the leaf of too much water. Further- 

 more, the chamber keeps the gases well distributed. Carbon dioxide is 

 brought alongside the cell walls, where they can absorb it, oxygen is car- 

 ried out. 



Look through the wall of this cell which forms the right-hand side of 

 the chamber. The wall is thin and almost transparent. Behind it you can 

 see the streams of protoplasm winding up and down and across, engaged 

 in transactions too subtle to be even guessed at. Far along the side you 

 can make out a globe, the nucleus, in which — if only our eyesight were 

 more acute — we could see the chromosomes that lie ready to procreate if 

 the signal comes. The central portion of the cell is full of sap, which is 

 under pressure, keeping the walls taut. Many "organs" of the cell are 

 vaguely visible — the technical names for which are confessions of igno- 

 rance about their function — chondriosomes, Golgibodies, cytoplasmic 

 granules, microsomes. Be at ease. I am not going to discourse on the un- 

 known. I merely wish it understood that you are gazing, not at specks of 

 stuff, but at the "organs" of a life which is far, far beyond comprehension. 



Most prominent among the contents of this cell are the green disks, two 

 or three feet in diameter, which float in the protoplasm. These are the 

 places where some red and blue rays of sunlight are set to work for the com- 

 pounding of water and carbon dioxide into sugar. If you were not so large, 

 you would not be blind to what goes on within the disks. You can see 

 nothing. Science has not yet been able to see anything. We might as well 

 move on. 



On to what? When you have clamped this gas-mask over your head and 

 wormed your way two hundred feet toward the axis of the needle (only 

 a third of the distance), you have passed a confusing array of cells of many 

 sorts and of all shapes, jammed together Hke a medley of elastic factory 

 rooms under hydraulic pressure, flattened here, bulged there, now almost 



