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 
which 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 
