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- 

 nned 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 



Fio. 125. Self-pollinated flowers flowers ' Merel y readin g 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 ou t of doors, and then their 

 other three bend inward until their ,. ,, . , 



anthers overhang the stigmas s ff , thus structures carefully examined 



making self-pollination extremely in the laboratory, 

 easy. Magnified about 6 diameters. T , , , 



After H. Mailer In the present chapter 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. 1 



135. Knotgrass 2 ; self-pollination. The common dooryard 

 weed known as knotgrass, knotweed, or door weed 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 



i See Knuth-Davis, Handbook of Flower Pollination, Vol. I. Clarendon 

 Press, Oxford. 2 Polygonum aviculare. 



