CHAP. iv.J REPRODUCTION OF PLANTS. 141 



orchids and insects has become so perfect that in the 

 great majority of species there is only one fertile stamen 

 present, yet the whole working is so exact that cross- 

 fertilization is even more certain than is self-fertilization 

 in the case of allied groups having the full complement 

 of six stamens, and even if cross-fertilization was in 

 itself of no importance, the advantage of being able to 

 dispense with five stamens alone would compensate for 

 the evolution from the primitive type, as the change has 

 not necessitated the development of entirely additional 



Fig. 41. Pollen-masses of an or- 

 chid. The upper thickened portion 

 consists of the pollen-grains stuck 

 together by a viscid gum-like sub- 

 stance that forms two slender stalks 

 terminating in a flattened portion 

 below. This flat part adheres to the 

 head of the insect. (Highly mag- 

 nified.) 



structures, but only a modification of parts already 

 present. 



The type of having the petals more or less grown 

 together to form a tube is well illustrated by the white 

 dead-nettle {Lamium album), where the five petals are 

 very irregular in shape and size, and grown together in 

 such a manner as to form two large lip-like portions, the 

 upper arched lip being formed of two petals, and 

 serving to protect the stamens and pistil from rain ; the 

 lower lip consists of three petals grown together, and 

 serves as a landing-stage for insects that visit the 

 flower. The stamens are four in number, the fifth pes- 



