80 TALKS AFIELD. 



of these tubes must reach each ovule before 

 the ovule can develop into a seed. In just 

 what manner the pollen tube acts upon the 

 embryo-sac of the ovule is not known. We 

 can make an ideal picture of these poUen 

 tubes as represented in Fig. QQ^ the figure 

 at the right representing a longitudinal sec- 

 tion of a portion of the stigma. M. Brong- 

 niart compared the appearance of a stigma 

 penetrated by pollen tubes to " a pin-cushion 

 entirely filled with pins stuck into it up to 

 the head." 



If we refer to our talk about the flower on 

 page 29 et seq.^ we can readily understand 

 how a flower which contains both stamens 

 and pistils, as the apple, is perfect^ and all 

 which do not contain both organs are imper- 

 fect. The greater part of our common plants 

 have perfect flowers. In many of our trees, 

 as the walnut, butternut, hickories, oaks, 

 chestnut, beech, and birches, the stamens and 

 pistils are borne in different flowers on the 

 same tree. Such plants are said to be mon- 

 wcious^ — the flowers are borne in " one 

 house." In the willow and some other 

 plants the staminate and pistillate flowers 

 are borne on entirely distinct plants, — in 

 "two houses," and such plants are termed 



