284 



SCIENCE. 



[N. S. Vol. I. No. 11. 



volume. We know, also, that this varia- 

 tion amongst the sisterhood or colony of 

 branches is determined by very much the 

 same conditions which determine variation 

 in independent plants growiug in soil. I 

 believe that the primary and most im- 

 portant determinant of this variation is the 

 variation in food supply, the same which 

 Darwin believed to be the most potent fac- 

 tor in the origination of variations in gen- 

 eral. That branch or phj^on which re- 

 ceives the most food, because of its position 

 or other incidental cu-cumstance, is the one 

 which gTOws the largest, has the heaviest 

 and greenest leaves, and, in the end, is the 

 most fruitful. I use the word food to desig- 

 nate not only the supply of nuti-iment 

 which is derived fi-om the soil, but also that 

 obtained irom the air and which is most 

 quickly and thoroughly elaborated in the 

 presence of the brightest sunlight. Thus 

 the uppermost branches of the tree, whilst 

 farthest from the root, are generally the 

 strongest, because they are more freelj^ ex- 

 posed to light and air and their course is 

 least impeded. Many bi'anches in the in- 

 terior of tree tops are undoubtedly parasites 

 upon the plant colony, taking from it more 

 than they return. 



If the number of the plant units is deter- 

 mined by circumstances peculiar to that 

 plant, and if there is variation amongst 

 these units in any plant, then it follows that 

 there must be struggle for existence between 

 them. And this struggle diifers from the 

 conflict between independent plants in the 

 complex battle for life only in the circum- 

 stance that it is more intense or severe, 

 from the fact that the combatants are more 

 closely associated. There are weak branches 

 and strong branches, and the survival of the 

 fittest is nature's method of pruniug. The 

 strong terminal branch, shootiug upwards 

 towards air and sunlight, makes the bole of 

 the tree, whilst the less fortunate or side 

 branches perish and fall. The leaf surface 



of any tree or large plant is always puslung 

 outwards towards the peripherj', which is 

 only another way of saying that the an- , 

 terior branches die. I often find fruit 

 growers who refuse to prune their trees be- 

 cause they believe it to be unnatural, while 

 at the same time theu" tree tops are full of 

 dead limbs, eveiy one a monument to the 

 stupiditj^ of the owner ! 



Now, the effect of this struggle for exist- 

 ence allows of mathematical measurement. 

 Each bud should produce a branch or a clus- 

 ter of fruit. A seedling peach tree maj"^ be 

 two feet high the fii'st year, producing thirty 

 leaves, and ui every axil a bud. Each of 

 these buds should produce a branch, which 

 should again produce thirty buds. The 

 third year, therefore, whilst the tree is only 

 six or eight feet liigh, it should have 900 

 branches, and in the fourth year 27,000 ! 

 Yet a peach tree twenty j-ears old may not 

 have more than 1,000 branches ! That is, 

 manj' millions of possible branches have 

 been suppressed or have died. I once 

 made an actual observation of such a battle 

 and counted the dead and wounded. A black 

 cherrj' tree came up near mj^ door. The 

 first year it made a straight shoot nineteen 

 inches high which produced twenty-seven 

 buds. It also sent out a branch eight inches 

 long which bore twelve buds. The little 

 tree had therefore enlisted thirty-niue sol- 

 diers for the comiag conflict. The second 

 year twenty of these buds did not grow. 

 Nineteen of them made an effort, and these 

 produced 370 buds. In two j'ears it made 

 an effort, therefore, at 409 branches, but at 

 the close of the second year there were only 

 twenty-seven branches iipon the tree. At 

 the close of the third year the little tree 

 should have produced about 3,-500 buds or 

 branch-germs. It was next observed in 

 Julj^ of its fourth year, when it stood just 

 eight feet high ; instead of having between 

 3,000 and 4,000 branches, it bore a total of 

 297, and most of them were onlj' weak 



