THE FLOWER 219 



pollen speck on the petal of a rose, to the brilliant staminodia, 

 or transformed stamens of the canna (Fig. 327), which simu- 

 late petals so perfectly that their real nature is never sus- 

 pected by the ordinary observer. The transition from spines 

 and bracts to the brilliant corolla of the cactus (Fig. 328) 

 is so gradual that we are hardly aware of it till we examine a 

 specimen and see it actually going on before our eyes. 



It must not be supposed, however, that an organ is ever 

 developed as one thing and then deliberately changed into 

 something else. When we speak loosely of one organ being 

 modified into another, the meaning is merely that it has de- 

 veloped into one thing instead of into something else that it 

 was equally capable of developing into. 



245. The course of floral evolution. — For the reasons 

 mentioned, the flower is regarded as merely a branch with 

 modified leaves and the internodes indefinitely shortened so 

 as to bring the successive cycles into close contact, the whole 

 being greatly altered and specialized to serve a particular 

 purpose. With this conception of the nature of the flower, 

 we can readily see that the less specialized its organs are and 

 the more nearly they approach in structure and arrangement 

 to the condition of an undifferentiated branch, the more 

 primitive and undeveloped is the type to which it belongs. 

 On the other hand, if the parts are highly specialized and 

 widely differentiated from the crude branch, a proportion- 

 ately high stage of floral evolution is indicated. 



V. FUNCTION AND WORK OF THE FLOWER 



Material. — For this exercise, flowers of the mallow family — holly- 

 hock, abutilon, mallow, hibiscus, cotton, okra, etc. — are particularly 

 recommended because they have pollen grains so large that they can be 

 studied fairly well with a hand lens. Lily, tulip, iris, etc., will also meet all 

 essential conditions of the study outlined in the text. A strand of silk 

 from a pollinated ear of corn is an excellent example for showing the 

 growth of the pollen tube, under the microscope. 



Appliances. — A compound microscope ; a watch crystal ; sugar solu- 

 tion of 5 to 15 per cent. 



