PHYSIOLOGICAL BOTANY. 



PART II. 



268 



len grains each protruding a tube and appearing like 

 pins on a cushion. In certain families, as the Orchi- 

 deae (Jig. 162. a) and Asclepiadea? (/;), the grains 

 contained in one cell of each anther are agglutinated 

 together into waxy masses, so that when the action 

 takes place, a number of tubes are 

 protruded together and form a thick- 

 ened cord (as at c) ; and thus 

 they penetrate into the <>\;irium "en 

 masse." Even some grains which 

 are composed of only one vesicle, 

 exsert more than one pollen tube. * 

 In some cases the tube originates 

 in a swelling on the surface of the grain, which then 

 seems to be formed of one skin only, or perhaps the 

 two may be united. 



THIRD PERIOD OF REPRODUCTION. 



(265.) Maturation. After the action of the pol- 

 len has taken place, the ovules contained in the ovarium 

 begin rapidly to increase, and the fruit swells and 

 ripens. But in order to understand the several parts of 

 which the seed is composed, it is necessary to trace 

 the changes which the ovule undergoes, from the 

 earliest period in which it is distinguishable in the 

 young flower-bud, up to the time when the complete 

 maturation of the fruit is effected. 



(2()6.) Origin of the Ovule. When the ovules can 

 first be seen (as in some 

 species of the cucumber or 

 gourd), they are small pus- 

 tules or wartlike excres- 

 cences formed upon the 



inner surface of a cavity in the ovarium; and are with- 

 out any distinct traces of organisation (fg- lG'3. ) 

 Soon after their first appearance we find them lengthen- 

 ing (/>), and assuming traces of an organised structure (c). 

 They are observed to consist of an internal mass of eel- 



