POLLEN AND PISTILS OF APPLES 401 



ment. They have very thin, cellulose walls which in some cases 

 contain pectin and their protoplasm consists of a very thin peri- 

 pheral layer enclosing a siing'le large vacuole. The sap filling the 

 vacuoles occasionallly contained small traces of cane sugar and 

 som'etimes substauces reacting to the tests for pectin. The 

 amount of pectin in the walls varied in different papillae on 

 the same stigma, some giving no evidence of any, while others 

 showed a distinct color reaction with ruthenium red and methy- 

 lene blue. There was no reaction with Sudan III, or Millon's 

 reagent. At the opening of the flower the protoplasmic layer had 

 pulled away from the wall at the apex of some of the papillae as 

 shown in figure 166. This shrinking continued in the open flow- 

 ers, and in a few days, ranging from three to six, when flowers 

 v^ere left exposed on the trees, the walls of the papiUge over the 

 upper surface of the stigma were completely collapsed and the 

 protoplasm was becoming brown. This death of cells moves rap- 

 idly down the style and in a period of a few days, both stigma 

 and style are dead and withered.. 



It is the reflection and refraction of light from the vacuoles 

 and from the outer surface of the plasmic membranes when 

 pulled away from the walls that cause the glistening which by 

 some is regarded as an indication that the stigma is receptive. 

 At this stage the papillfB would not become turgid when water 

 was supplied and they showed no change when immersed in. 

 strong salt or sugar solutions. Judging from the behavior ot 

 their protoplasms toward solutions, the papiHaB are dead at the 

 period of pollination. The brown color which the stigma soon 

 takes on after the flower opens is further evidence of the early 

 deatli of the papillae. 



The styles and stigmas are much more sensitive to cold than 

 the pollen. As indicated by taking on a broiwn color and becom- 

 ing dry and brittle very soon, the styles were killed by an ex- 

 posure of an hour to a temperature of — 1° C. 



THE! SECRETIONS AND THE GERMINATION OF THE POLLEN ON 



THE STIGMA 



The stigma is' commonly considered a secreting organ, and the 

 notion is prevalent that stigmatic secretions have much to do 

 with the germination of the pollen. Stigmatic secretions may 

 have much to do with the germination of pollen in many plants, 

 but very likely too much has been attributed to them. 

 26 



