164 HORTICULTURE FOR SCHOOLS 



understood. (2) The style must be of the proper length so 

 that the tip of the pollen-tube will just reach a point near the 

 micropyle of the ovary at the time the tube is ready to dis- 

 charge the pollen-cell which will fertilize the ovule. On this 

 account pollen-grains which normally produce tubes shorter 

 than the length of the style cannot successfully fertilize the 

 ovule. In some instances, pollen-grains normally produce 

 pollen-tubes much longer than the style, and the tip of the 

 tube grows past the micropyle before being ready to dis- 

 charge the cell to fertilize the egg-cell. In either of these 

 cases, fertilization cannot take place. (3) The tissue of the 

 style must be of such structure that the pollen-tube can 

 penetrate it. Sometimes the style possesses a stylar canal, 

 an opening down which the pollen-tube makes its way, but 

 often the tube grows down through the tissues, either forcing 

 its way between the cells, or actually dissolving some of them 

 and absorbing the dissolved material to aid in its own growth. 

 (4) It appears that the cell of the ovary must have a certain 

 attraction or affinity for the pollen-cell, otherwise fusion of 

 the two does not occur. This attraction is probably chemical 

 in nature, but is not well understood. (5) Weather, condi- 

 tions may be favorable or unfavorable in relation to the pistil. 

 Sunshine is generally favorable, especially by rendering it 

 possible for bees to carry pollen from flower to flower. 

 Rainy weather is unfavorable to mature stigmas, inasmuch as 

 the water washes away some of the stigmatic secretions. 

 Dry hot winds tend to cause drying of the stigmas. 



261. Transportation of pollen is of great importance. In 

 some of the lower forms of plant life, the little sperm, corre- 

 sponding to the pollen-cell of higher plants, is motile and 

 can swim through films of moisture to the egg-cell of the 

 female organ, but in higher forms some outside agency must 

 be depended on for bringing the pollen to the stigmas of the 

 pistils. There are three main transporting agencies : water, 

 winds, and insects. 



