CORRELATION OF FACTORS IN ORGANIC GROWTH. 397 

 CORRELATION OF FACTORS IN ORGANIC GROWTH. 



By Herb EDUARD STRASBURGER. 



THE uniform co-operation of living cells in tlie vegetable or- 

 ganism appears less problematical to us when we know that 

 these elements are connected by fine threads of living substance. 

 These protoplasmic threads penetrate the cell walls; they im- 

 mediately transmit the stimulus from cell to cell, and conduct it 

 to a distance ; and the continuity of the living substance in the 

 whole organism is thus preserved. It formerly appeared other- 

 wise, when the single living cell bodies were supposed to be com- 

 pletely separated by their walls, and these cell walls were thought 

 to bring about the transmission. The physiological arrange- 

 ments of plants have now become very much like those of ani- 

 mals, and nearly approach them in perfection. Very striking 

 among the life expressions of organisms are certain processes 

 which mutually influence and condition one another, and which 

 we call manifestations of correlation. A particular condition in 

 the organism invokes another, so brought about that a general 

 balance in the functions is preserved, and is restored if disturbed. 

 A red-beech tree growing in the open, where it is immediately 

 exposed to the effects of the light, has small but relatively thick 

 leaves. Red beeches, as undergrowth in the shadow of the woods, 

 are distinguished by considerably larger but thinner leaves. The 

 cause of this variation lies chiefly in the difi^erence in the condi- 

 tions of transpiration. The growing leaves in the isolated tree 

 give out more vapor to the atmosphere than do those in the 

 shade. The increased evaporation afl:ects the structure of the 

 leaf surfaces, and they are compressed, chiefly because less air 

 space is found between their cells, and partly, also, because the 

 cells turn perpendicularly to the surface instead of increasing in 

 breadth. AU this increases the thickness of the leaf at the cost 

 of its superficial diameter. This condition is immediately useful 

 to the plant, because a thin and comparatively broad leaf surface 

 would transpire too rapidly in an open situation, involving the 

 tree in danger of drying out. In the shade, on the other hand, a 

 large surface is necessary to give as much vapor out to the at- 

 mosphere as the life processes of the plant require ; for evapora- 

 tion promotes the accession of food-salts from the soil. These 

 salts, dissolved in water, reach the plant, and are retained by it, 

 while the water is evaporated. More rays of light fall upon a 

 large leaf than upon a small one. In this view too, then, an en- 

 largement of the leaf surface is more advantageous and useful in 

 a feebly lighted situation. The intense light which falls upon 

 a fully exposed beech maj^, in its greater intensity, perform as 



