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A TEXTBOOK OF THEORETICAL BOTANY 



.TV 



Most pollen readily absorbs water, so it may be spoilt by moisture and it 

 is important to protect it from damp. Moreover in damp weather the distri- 

 bution of pollen by wind will be less effective. Various methods are known 

 which prevent the discharge of pollen in wet weather or preserve it in a dry 

 state if it is shed. The anthers of some plants, for example, open only 

 in dry air. The pollen of many catkin-bearing trees is not actually scattered 

 when it leaves the anther. As the pollen is discharged it accumulates in 

 the flowers in a position protected from moisture and is subsequently 

 scattered by the shaking of the catkin by the wind. In a similar manner the 

 pollen of various Conifers collects on the under side of the stamens and it is 

 from this that it is finally distributed. In Hippophae rhamnoides the pollen 

 is concealed in two shell-like scales which meet at the top and open only at 

 the sides in dry weather. In Triglochin maritimiim the pollen collects in 

 boat-shaped pockets situated under the anthers and it is from here that it is 

 scattered by the breeze. (See also p. iioi.) 



{a) Flowers with no Stigma 



In this group are included the Gymnosperms, in which no stigma 

 exists and the pollen grains find a way between the scales of the female 

 cone to the ovule itself. The pollination of Pimis and Taxus has already been 

 described in Volume I. 



Although the possession of a stigma is one of the distinguishing marks 

 of the Angiosperms, there is at least one species in this class whose stigma 

 is functionless and in which pollination is directed to the ovule as in Gymno- 

 sperms. This is Rheum aiistrale (Polygonaceae), which has the usual three 

 large stigmas of its genus. There is only one, erect ovule, whose nucellus 

 protrudes through the micropyle of the integument and also through the 

 apex of the ovary. Thus exposed it becomes adhesive and receives pollen 

 directly, the stigmas taking no part in the process (Fig. 12 16). 



Fig. 1 2 16. — Rheum aiistrale. A, Vertical section of the gynoecium with open top 

 from which the apex of the soHtary ovule protrudes. B, Ovule with nucellus 

 protruding from the micropyle. C, The gynoecium with three functionless 

 stigmas and the protruding apex of the nucellus, bearing pollen grains. {After 

 Velenovsky.) 



(b) Flowers with Stigma 



It is to this group that all the wind-pollinated Angiosperms belong. 

 There are a number of different types which are best treated separately. 



