476 AMERICAN JOURNAL OF BOTANY [Vol. 8, 



of the style frequently are loosely connected, elongated, and sometimes 

 mucilaginous. The pollen tubes, according to most histological reports, 

 penetrate the middle lamellae of these cells. This is the condition most 

 frequently met with. The pollen tubes follow the middle lamellae of the 

 cells throughout their course. The lamellae are, as has already been stated, 

 composed either of pectin or of closely related mucilaginous substances. 

 Here again the necessity for a pectin-digesting enzyme is evident. It has 

 been sought for in the experiments reported later. Since this condition is 

 the most common, many examples could be cited. It may be well seen in 

 members of the grass family and in Salvia (Bower, 191 9, p. 269). Histo- 

 logical evidence seems to indicate that the cells of the style often remain 

 intact. Shreve (1906, p. 115) says in regard to the pitcher plant {Sarracenia 

 purpurea) : 



The pollen tubes grow between the cells of the stigmatic surface and their entire passage 

 is between the cells of the conducting tissue and never through them. 



Gow (1907, p. 136), describing the fertilization of skunk cabbage {Spathy- 

 ema foetida) , writes: 



The central portion of the style consists of a loose mass of thin-walled cells through 

 which the pollen tube readily forces its way to the upper end of the ovary. 



Miller's account of the growth of the pollen tube of corn through the 

 silk or style is interesting (1919, p. 264): 



Each silk has two fibro-vascular bundles. These bundles are surrounded by sheath 

 cells which are characterized by their dense contents and large flattened nuclei. It is 

 between these cells that the pollen tube travels down the silk. Arriving at the base of the 

 silk the pollen tube works its way between the sheath-like cells that extend from the fibro- 

 vascular bundles of the silk to the cavity of the ovary. The tube enters the ovary and 

 twists and coils in its passage along the ovule coat until it reaches the micropyle. The 

 pollen tube then pushes between the cells of the ovule until it reaches the embryo sac. 



Again, in another part of his account, he says: 



The end of the pollen tube is greatly enlarged as it pushes its way between the sheath 

 cells of the bundle. In its passage down the silk the tube causes but little disturbance in the 

 position of the cells, so that after the tube disappears the cells quickly return to their normal 

 form and position. [The emphasis here is my own.] The pollen tube so far as I have 

 observed does not extend the full length of the silk at any time. It is difficult to locate it 

 a short distance back of its growing region. It appears that the older portions of the tube 

 are absorbed by the surrounding cells, while the growing part of the tube is apparently 

 nourished by the dense sheath cells. 



Land (1907, p. 276), in explaining the fertilization of Ephedra trifurca, 

 notes that the pollen tubes force their way between the neck cells of the 

 archegonium, rarely destroying them in their passage. Only in two in- 

 stances were the lower neck cells destroyed. 



(3) Cell xvalls penetrated by pollen tubes. According to most investigators 

 this condition occurs only rarely. Perhaps it will be found more frequent 

 if more observations are made. The classic illustration is corn cockle, 



