STUDIES ON Ti-iE FERTILIZATION OF PLANTS. 213 



before it has ripened and after it has dehisced or opened, setting the 

 pollen free. The meally yellow dust that is shed so freely at the slightest 

 touch is the pollen. Shaking the branch sets free a perfect cloud of 

 it. It is borne off in the air like a whiff of smoke. Only one pollen grain 

 is necessary to fertilize one ovule. Many pollen grains will of necessity, 

 whatever be the mode of fertilization, fail to reach their intended place 

 upon a ripe stigma. The ratio of pollen grains to the ovules must 

 always be large. The night blooming Cereus has 250,000 pollen grains 

 to 30,000 ovules, or about 8:1. The garden wistaria has a ratio of about 

 7,000 to 1 ovule. The Indian corn, pines and other wind fertilized flowers 

 must have a much greater ratio than this. 



One can see at a glance (Fig. 6) that the pollen grains of the oak, be- 

 cause of their position, stand little chance of falling upon the stigma of 

 the branch upon which they are borne. It would require an upward 

 draft of air or an insect or other animal to carry them. It would be 

 easier to account for the transfer of the grain of pollen in this case by sup- 

 posing it to have dropped gently from the boughs above. It may as well 

 have been carried by a light breeze from -a neighboring tree. 



There is neither odor, nectar, edible pollen, nor showy corolla, to 

 guide or attract a busy insect, and since all insects seem bent on business 

 they would spend little time loafing around the oak blossom. In fact it 

 would be a one-sided bargain for an insect to carry pollen for the oaks 

 since he would derive no benefit to himself. Wherever a relationship is 

 discovered between a plant and an animal it may be taken as axiomatic 

 that the association is mutually beneficial. Darwin once staked his theory 

 of organic evolution upon the proposition that if any organ or modification 

 of an organ could be found in the animal or plant world that was present 

 wholly for the benefit of another species, that he then must admit that 

 his whole conception might be based upon false conclusions. Fifty 

 years have passed and no one has produced the evidence. 



Assuming that a pollen grain has found its place upon a sticky stigma 

 of a pistillate flower let us see what takes place. The grain of pollen 

 absorbs moisture and swells until it begins to grow a tube, somewhat as 

 a se<?d sends down its radical. It either enters a space left between the 

 cells or by penetrating the cells grows until it reaches the generative cell 

 of the ovary. An interesting series of experiments has been made show- 

 ing the cause of growth down the style to the ovule to be chemotaxis, or 

 growth toward chemically attractive substance. The essential part of 

 the pollen liquid now penetrates the ovule to the nucleus of the genei'a- 

 tive cell. There immediately follows an interesting series of phenomena 

 of especial interest to the embryologist. In brief, the one cell subdivides 

 many times and grows ultimately into an acorn, which one year later will 

 be recognized as such a one as now appears on last year's growth. The 

 season's growth increases it to the normal size and in September or Oc- 

 tober it is ripe and ready to leave the tree, and soon finds its resting 

 place upon the ground. (The white oak and some others mature their 

 acorns in one season. Not all the pistils are fertilized. Some of the 

 acorns fail to grow the second year.) The fallen acorns roll about or 

 are kicked or carried about by animals. Squirrels bury them at some 



