72 



PLANTS AND MAN 



surrounded by showy leaf-like bracts, purple or red in color, 

 which are often mistaken for petals. Dogwood trees have con- 

 spicuous white bracts beneath the clusters of small, yellowish 

 flowers; while the poinsettia "flower" is in reality red leaves 

 beneath an insignificant group of yellowish flowers. In the jack- 

 in-the-pulpit (fig. 45) and the calla lily, the flowers lack both 

 petals and sepals; in the former case a green and brown bract (the 



pulpit) surrounds and arches over 

 the flowers; in the latter, a broad 

 white bract spreads out beneath the 

 base of the axis bearing minute 

 flowers. 



Reproduction begins with the 

 formation of the microspores and the 

 megaspores, as in the Gymnosperm. 

 Using the apple tree as an example, 

 we see that the flowers have unfolded 

 an arresting set of petals which suc- 

 ceed in attracting insects in search of 

 honey. The microspores (fig. 46) 

 develop in sporangia which appear 

 as sacs in the stamens, the latter 

 corresponding roughly to the scales 

 of the male pine cone. When mature, 

 the spores are released through 

 I breaks in the tip of the stamen, so 

 that they frequently form yellow 

 powdery masses on the outside of the 

 stamen. While the apple blossom is 

 expanding its fragrant corolla as an 

 open invitation to the insect world, 

 the contents of the pollen (or mature 

 microspores) are undergoing changes 

 of an important nature. Male nuclei develop, taking the place of 

 motile sperm or male cells; there is no trace of the male prothallus 

 cells such as there is in the pine. Thus when the pollen is shed, 

 each grain has passed into the male prothallus stage with its 

 mature male "cells" in the form of nuclei. 



Fig. 45. — In some cases, 

 in the Jack-in-the-pulpit, addi- 

 tional structures are pressed 

 into service to attract insects; 

 the "pulpit" is a special form 

 of leaf known as a spathe. 



