474 AMERICAN JOURNAL OF BOTANY [Vol. 8, 



The pollen grains are very resistant to excessive heat, cold, or dryness, 

 and certain kinds retain their viability for many years. The pollen of the 

 date palm tested by Popenoe at the Mecca experiment station was kept 

 seven years and still retained its power of germination. Goodale (1916) 

 found that dry pollen could retain its active poisonous properties for twenty- 

 five to thirty years. It is evident that pollen is an interesting physiological 

 unit, and our knowledge of its composition should be more complete. 



Since one cell of the pollen grain is vegetative and gives rise to the pollen 

 tube, food must be stored in the grain and at the time of germination ren- 

 dered available. We should expect therefore to find enzymes suitable for 

 the digestion of the materials stored in the grain, and perhaps capable of 

 also digesting the inner pectin membrane (Mangin, 1893, p. 655) which 

 envelops the grain. It is one aim of the experiments reported to determine 

 whether such a correlation exists. 



The distance that the pollen tubes have to travel se varies greatly. 

 Where a style is absent and the stigmatic surface is just above the ovary, 

 as in Vitis and Actaea, the tube has only a little way to penetrate. In 

 flowers with long tubular corollas and slender filamentous styles, such as 

 Crocus, Oenothera, and Zea Mays, the tubes attain a relatively great length. 

 The time required for them to reach the ovule also varies greatly. In some 

 flowers the tube reaches its full development in a few hours, while in the 

 pine, following pollination in the spring, the grains put forth short tubes 

 v/hich do not complete their growth for a year (Kerner, 1895, 2: 420). In 

 certain oaks thirteen months elapse between pollination and fertilization. 

 In regard to the Taxaceae, Coulter (1910, p. 268) writes: 



The tube may advance directly toward the archegonia or it may pursue a devious 

 route, in some cases not reaching the archegonia until during the second season. 



Other instances are cited by Coulter and Chamberlain (1903, p. 147). 

 Why this long delay? An interesting physiological and chemical problem 

 is waiting to be solved. The 13-inch pollen tube of Colchicum autumnale 

 needs only twelve hours to reach its goal, and the 9-inch tube of Cereus 

 grandiflorus completes its growth in a few hours (Schleiden, 1849, p. 407). 

 In Iris versicolor the male nuclei were observed in the embryo sac 79 hours 

 after fertilization and the tubes were 14 mm. long (Sawyer, 191 7, p. 163). 

 Surely an intruding, growing tissue of such size and duration must during 

 its period of development, profoundly affect the cells with which it comes 

 in contact, or which are adjacent to it, in its passage through the style. 

 It has long been customary to liken the pollen tubes to the haustoria of 

 parasitic fungi, for they closely resemble the latter in many respects. In 

 Pinus, according to Mottier (1904), the tube serves both as a conducting 

 passage for the male gamete and as an absorber of nutriment. The haus- 

 torial habit seems to be the more primitive condition, and we have survivals 

 of it in certain Angiosperms, as in Iris versicolor (Sawyer, 1917), hazel, oak, 

 elm, hickory, and certain mallows (Kerner, 1895), where the tube branches 



