FERTILIZATION. 167 



Fig. 70. 



see through their thin transparent walls the movements of the 

 microscopical corpuscles which it contains. 



As soon as the pollenic tubes have been protruded from the 

 pollen grain, they penetrate the loose cellular tissue which 

 constitutes the mass of the stigma, known as the conducting 

 tissue, and insinuating themselves amongst the interspaces of 

 its cells where they find an abundance of moisture, they grow 

 downwards through the central part of the style until they 

 reach its base, a distance in some cases of several inches. 

 Hence by making a longitudinal section of the pistil we are 

 able to find these tubes and to trace their course. 



The pollen tubes may be readily inspected under the micro- 

 scope in many plants, and in none more readily than in the 

 Asclepias or milkweed. In that family the pollen grains 

 cohere together in masses termed pollinia, and their united 

 tubes are protruded, and consequently are of such a size as to 

 be easily perceived with a very moderate magnifying power. 

 Fig. 71. 



The action of the pollenic tubes on the ovules. At the time 

 that fecundation is operating, and the pollenic tube is being 

 elaborated, the ovules are organized into a suitable form for its 



