76 LETTER XXXlf. 



pull open the sides of that case ; to enable their forces to 

 act with certainty, the sides of the anther are weakened 

 in a particular line, which in every anther of the same 

 species is constantly the same. It is supposed that 

 the clamminess of the stigma is not merely to stick 

 the pollen-grain fast, but also to cause the formation 

 of the pollen-tube ; to enable the latter to reach the 

 ovule, notwithstanding its excessive delicacy, the whole 

 texture of the stigma and style is loosened, so as to 

 offer as little resistance as possible to the passage of 

 the pollen- tube. In this Rock- Rose Tribe we have 

 a still further example of the facility with which ob- 

 stacles to communication between the pollen-tube and 

 the opening in the ovule are overcome. 



If we suppose a grain of pollen to fall on the stigma 

 of aCistus (fig. 3. a.), its tube may be easily under- 

 stood to reach the place where the ovules grow 

 (fig. 3. h.) ; but, when there, it is cut off from the 

 foramen by the whole length of the stalk and sides of 

 the ovule, for the foramen is at the other end of the 

 latter. In order to overcome this difficulty, we are told 

 bv M. Adolphe Brongniart, that the pollen-tube does 

 not follow the placenta till it reaches the ovule (at 5.), 

 but quits the style at the top of the cavity of each 

 cell (c), and thence lengthens in the open space inside 

 the ovary, in the form of the finest imaginable cobweb, 

 till it reaches the foramen in the end of the ovules. 



To make this clearer, observe the following dia- 

 gram (fig. 10.). Let the perpendicular a. g. represent 

 the style, the line a. h. the side of the ovary, the hori- 

 zontal line b. c. the base of the ovarv, the curve a. d. 



