L894.] Pollen Grain and the Nutrition of the Pollen Tube. 127 



riew is derived from some experiments on the pollen of Zamia. A 

 itery extract of this pollen was found to have no diastatic power, 

 it on being warmed with a little malic acid for some hours, and 

 then neutralised, it was found to have acquired a feeble one, very 

 slowly hydrolysing some thin starch paste. Further experiments 

 upon this point are, however, necessary before pronouncing decidedly 

 that the zymogen exists. 



The whole of the researches may be summarised as under : 



1. Diastase and invertase are both present in pollen grains, and 



can be extracted from them by the same treatment as has been 

 found effectual in the cases of seeds and foliage leaves. The 

 relative quantities vary a good deal ; while some pollens con- 

 tain both, others possess only one, which may be either of the 

 two. 



2. At the onset of germination the amount of both diastase and 



invertase is usually considerably increased. In one species 

 examined this increase was preceded by a primary diminution. 

 When the pollen grain has lost the power of germinating, the 

 quantity of diastase has considerably decreased. 



3. The pollen tube is nourished during its growth by 'plastic 



reserve material derived from two sources, the store of material 

 in the grain itself, and a further store deposited in the style. 



4. The reserve store of the pollen grain consists of different 



materials in different species : starch, dextrin, cane sugar, 

 maltose, and glucose being the forms in which it is found. 



5. The store in the style consists usually of the same carbohydrates, 



with the exception of dextrin. 



6. The style itself contains enzymes to assist in preparing the 



reserve materials for absorption by the pollen tube, while the 

 latter excretes the same ferments during its progress down 

 the conducting tissue. 



7. The absorption of food material appears to be one cause of the 



increase of enzyme found to occur during the germination. 



8. This absorption of food material is usually so active that the 



reserve store of the pollen grain is often largely increased by 

 a temporary deposition, either in the grain or its tube, of 

 some of the absorbed sugar in the form of starch. 



9. There is a certain amount of evidence pointing to the existence 



of zymogens in some pollens, particularly such as germinate 

 in a faintly acid medium. 



