CHAPTER ELEVEN 



SEX 



i. NEARLY all familiar animals are bisexual; that is The two 

 to say, they have two sexes, male and female. The sexes 

 sexes may be so similar in appearance that they cannot 

 be distinguished without close examination ; or they 

 may be so different that it is hard to find any characters 

 in common. Among the. lowest animals we cannot dis- 

 tinguish sexes ; all the individuals are substantially 

 alike, and if they conjugate, it may be impossible to 

 regard one or the other as male or female. Sometimes 

 there is a difference in size, and then the smaller cell is 

 thought of as male, the larger as female ; but this is 

 only a rather loose analogy. Among the higher plants Sex of 

 we have no trouble in recognizing sex, though the sex phu 

 phenomena are in many respects quite unlike those of 

 animals. An ordinary flower, such as a buttercup or a 

 rose, has stamens and pistils. At the top of each stamen 

 is an anther, and when this bursts at maturity, the yel- 

 low powdery pollen is set free. This pollen consists of 

 grains, which are not gametes or germ cells, but which 

 produce such. At the bases of the pistils are the ovules, 

 and these again are not germ cells, but are the producers 

 of them. The pollen, falling on the pistil, grows a pollen 

 tube, which conveys the gametes to the ovule, to meet 

 the gametes there developed, and fertilization takes 

 place. Such flowers are neither male nor female, but 

 they produce structures which take on true sexual 

 functions. 



In many cases the stamens and pistils do not occur 

 together in the same flower. They may be borne by 

 different plants, which, as in the case of the willow, 

 present a quite different appearance when in flower. 



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