MONOCOTYLEDONS AND DICOTYLEDONS 259 



dons. The peppers seem to represent the simplest of the 

 Dicotyledons, and this great line may have begun with 

 some such forms. 



A very interesting fact in connection with the fertiliza- 

 tion of certain amentaceous plants has been discovered. 

 In birch, alder, walnut, hornbeam, and some others, the 

 pollen-tube does not enter the ovule by way of the micro- 

 pyle, but pierces through in the region of the base of the 

 ovule and so penetrates to the embryo-sac (Fig. 242). As 

 the region of the ovule where integument and nucellus are 

 not distinguishable is called the chalaza, this phenomenon 

 is known as chalazogamy , meaning "fertilization through 

 the chalaza." 



139. Buttercups and their allies. — This is a great assem- 

 blage of terrestrial herbs, including nearly five thousand 

 species, and is thought by many to be the great stock from 

 which most of the higher Dicotyledons have been derived. 

 The alliance includes the water-lilies, buttercups, and pop- 

 pies, the specialized mustards, and certain notable tree 

 forms, as magnolias, custard-apples, and the tropical laurels 

 with one thousand species represented in our flora only 

 by the sassafras. Here also is the strange group of "car- 

 nivorous" plants {8arrace7iia, Drosera, Dioncea, etc.). The 

 group is distinctly entomophilous, in striking contrast with 

 the preceding one. 



Taking the buttercup {Ranunculus) as a type (Fig. 202), 

 the flower is hypogynous, the calyx and the corolla are dis- 

 tinctly differentiated and actinomorphic, and adajited for 

 insect-pollination, but the spiral arrangement and indefinite 

 numbers are very apparent, notably in connection with tlie 

 apocarpous pistils, which are very numerous upon a promi- 

 nent receptacle, but involving more or less all the parts. 

 The stamens are also very numerous (Figs. 200, 243, 244). 

 In the water-lilies the petals and stamens are indefinitely 

 numerous (Fig. 203), and in the poppies there is no definite 

 number. In many of the forms, however, in connection 



