POLLINATION AND FERTILIZATION OF FLOWERS. 221 



As already suggested, the pollen-grain is provided with a 

 liberal supply of reserve food materials, which during the 

 process of germination are changed and made available for the 

 nutrition of the growing tube. Nor is this supply of food mate- 

 rials confined to the pollen-grain, for the connective tissue of the 

 style is also rich in starch, sugar, maltose, etc. The absence of 

 chlorophyll and the abundant supply of elaborated food mate- 

 rials in the connective tissue, render probable an extra-cellular 

 digestion, depending on the active presence of certain enzymes. 

 It has repeatedly been shown that germinating pollen grains 

 have the power of reducing cane sugar,* while the pollen of 

 certain species growing in weak starch paste has been found to 

 liquify the paste and form maltose. t 



J. R. Green of Kew,| made an elaborate investigation of the 

 subject in 1893-4. Nearly all pollen examined by him was 

 found to contain diastase. Among the genera named are 

 Lilium, Helleborus, Helianthus, Gladiolus, Anemone, Antir- 

 rhinum, Tropseolum, Pelargonium, Crocus, Brownea, Alnus, 

 Tulipa and Cliyia. Invertase was found in Helleborus, Nar- 

 cissus, Richardia, Lilium and Zamia. "During the germination 

 of the pollen, the quantity of both enzymes was found to be con- 

 siderably increased ; in some cases four or five fold."§ 



But what is the value of all this discussion concerning 

 enzymes? As we know, the tube must make its way to the 

 embryo sac by penetrating the intervening tissue. The enzymes 

 appear to have the double office of dissolving the tissues in the 

 vicinity of the tube, and of acting upon the nutritive materials 

 as already suggested. The fact that in certain species the 

 pollen tube goes between the cells, burrowing through the mid- 

 dle lamella instead of penetrating the cell walls, has led to the 

 suggestion of a cytolytic enzyme, not yet demonstrated. Stras- 

 burger 1 1 mentions this point in connection with several genera 

 of Caryophylaceas and Malvacese. The presumption in favor 



* Van Tieghem: "Inversion du Sucre tie Canne par le pollen," RuL Soc. Bot. ile 

 Frence, Vol. 33, p. 216, 1SS4. 



jStrasburger: "Uber fraudartige Bestaubung." Jabrb.f. Wiss. Bot., Vol. 17, p. 04, 

 (1896) . 



t Green : "On tbe germination of pollen and tbe nutrition of the pollen-tube." 

 Phil. Trans. Roy. Soc., )S94, p. 3t^5. 



§1. c, p. 3S7; also abstract, Ann. of Bot., VIII, iiG, (1S!)4). 



II Strasburger: Befrucbt bei den Phan. Cited by Green, 1. c. 



