Ch. VI, 7] SIGNIFICANCE OF SEX 307 



earlier section (page 269). Here fertilization is effected, not 

 in water by a free-swimming sperm cell, as in all earlier 

 stages, but in the aii' by wind- or insect-carried pollen grains 

 from which the pollen tubes carry the sperm cells to the egg 

 cells. In correspondence with the dry and exposed sur- 

 roundings, the egg cell is deeply buried within the body of 

 the parent plant, — within an embryo sac, inside an ovule, 

 enclosed by an ovary, while the pollen occurs in closed an- 

 thers. Now the mode of transport of the pollen, by external 

 agencies, requires that the anthers, with some part of the 

 ovary fitted to receive the pollen, be accessible to wind and 

 insects ; and such is the function of stamens and pistils. 

 Accordingly these parts, speciall}' fitted to bring the sex cells 

 together, constitute physiologically the sexual organs of the 

 plant, even though on morphological grounds this designa- 

 tion has been denied them. Here is evidentlj- represented 

 stiU another stage in the evolution of sex, consisting in the 

 presence of sexual organs, fitted to effect union of the sex cells. 

 VII. In most plants the stamens and pistils are borne 

 close together in the same flowers, which are said to be 

 PEHFECT (or hermaphrodite). In some cases, however, hke 

 Birches and Oaks, they are borne in separate flowers on the 

 same plant, when they are said to be moncecioxjs. In anj^ 

 case only the stamens and pistils show structural differences 

 connected with the different sexes of the cells they produce, 

 and the plant itself shows no trace of sex. In a few kinds of 

 plants, however, the staminate and pistillate flowers are 

 borne upon separate plants (are dioecious), in which case 

 the plants are somewhat naturally, though not quite cor- 

 rectly, described as male and female. Ordinarilj' there is 

 no determinable difference, aside from the flowers, between 

 such plants, but occasionaUy, as in Date Palm, some Wil- 

 lows, and a few others, there is a marked difference in as- 

 pect between staminate and pistillate individuals, thus giving 

 a structural basis to the terms male and female as applied 

 to plants. Here, however, is the hmit reached by plants in 



