Dec, 1921] 
PATON — POLLEN AND POLLEN ENZYMES 
475 
frequently and serves apparently as both haustorium and directing channel. 
(See also Coulter and Chamberlain, 1903, p. 148.) The nature of the tube 
has been dwelt upon here at such length in order to emphasize the fact that 
we ought to know more fully how these tubular filaments make their way 
through the tissues of the style and ovary. We assume that they digest 
their way. One author of a recent textbook even states positively: 
Very soon after pollination, the tube cell begins to develop a pollen tube, which secretes 
an enzyme that dissolves the cell walls and contents of the nucellar tissue, thus facilitating 
the passage of the delicate tube. 
Is this true? Can we prove the existence of a cytase which digests 
the cell wall? Is one enzyme sufficient to account for the varied needs of 
the pollen tube in the course of its life history? 
There are several conditions which the pollen tubes may encounter 
before they reach the embryo sac. These are as follows: 
(1) An open stylar canal. In such cases the germinating tubes may 
force apart the cells of the stigma and soon enter the open space of the 
style without having to penetrate any cells, at least not until they reach the 
ovule. The middle lamella is usually composed of pectin compounds 
(Fremy, Mangin, Allen, and others). A pectin-digesting enzyme might 
therefore be required to dissolve the middle lamellae of the stigmatic cells, 
but afterwards the tube has a clear course. Examples of this sort are seen 
in violet, mignonette, lily, rhododendron, Hypericum, Cistus, Atropa bella- 
donna, and iris. According to Kirkwood (1906), in the Cucurbitaceae 
The tubes pass chiefly over the surface of the conducting tissue lining the stylar canal 
and covering the placenta lobes, and this is rich in starch. 
The suggestion is made that the tube is directed in its course by nutrient 
substances secreted by the conducting tissue. This would imply the pres- 
ence of a diastase to digest the starch. Even if there is actually no tissue 
to be digested, it seems reasonable to suppose that the tubes may derive 
nourishment from the cells lining the stylar canal. Negative aerotropism, 
positive hydrotropism, and positive chemotropism, which have been fre- 
quently demonstrated in pollen tubes, direct their course so that they 
penetrate the stigma. These same responses tend in many cases to keep 
the tubes closely appressed to the cells lining the canal. Considering the 
length of time it often takes a tube to reach the ovule and its considerable 
growth, enzymes along with other factors in nutrition must play an impor- 
tant part. Frequently, as in Anagallis, the channel is only a narrow space 
almost completely filled with a mucilaginous substance, supposed to be 
secreted by the cells lining the canal. It may be pointed out here that the 
mucilages are closely related to the pectins. If this material is utilized by 
the tubes during their passage through it, we should expect a suitable 
enzyme to be present. 
(2) A mass of loose, conducting tissue in the style. The cells in the interior 
