142 INTRODUCTION TO BOTANY 
It is worth while to mention the fact that the characters of 
the cleistogamous flowers of some violets are so sharply de- 
fined that they are of much use in enabling the botanist to 
distinguish one species from another. 
134. Variety of means for pollination. The details of the 
process by which some kinds of pollination are secured are 
most complicated. It has taken the studies of many botanists, 
based often on thousands of observations and carried through 
a lifetime, to work out our present fairly exact knowledge of 
most of the methods. Beginners 
in botany, in a school course, 
can hardly do more than follow 
in a very few instances some of 
the steps of the original inves- 
tigators of the pollination of 
Fic. 125. Self-pollinated flowers fowers. Merely reading about 
of knotgrass (Polygonum aviculare) the processes is not enough; 
A, top view; B, lengthwise section. the flowers must be watched 
Five stamens bend outward and the out of doors, and then their 
other three bend inward until their fs 
anthers overhang the stigmas sg, thus structures carefully examined 
making self-pollination extremely in the laboratory. 
easy. Magnified about & digmenets: Te: She icecenk chanier ant 
After H. Miiller Pp prer, only’ 
about ten floral types will be 
briefly discussed, out of nearly thirty under which flowers 
have been classed with reference to their form and mode of 
securing pollination.! 
‘135. Knotgrass”; self-pollination. The common dooryard 
weed known as knotgrass, knotweed, or doorweed is one of 
the best examples of a plant with flowers suited only for self- 
pollination. The flowers (fig. 125) are very small (often not 
as large as the head of a pin), greenish, and borne singly in 
the axils of the leaves. They are destitute of nectar and with- 
out odor, so they do not attract insects. There are usually 
eight stamens, five outer and three inner ones; these latter, on 
1 See Knuth-Davis, Handbook of Flower Pollination, Vol. I. Clarendon 
Press, Oxford. 2 Polygonum aviculare. 
