CHAPTER XXVI 

 THE ANGIOSPERMS 



The angiosperms as a whole possess features which separate 

 them sharply both reproductively and vegetatively from gymno- 

 spermous groups. The first of these in importance is the phenom- 

 enon of angiospermy itself. In the group the microspores or 

 pollen grains no longer reach the micropylar canal at the apex of 

 the ovule, but are accommodated on the tip of the megasporophyll, 

 which becomes modified as the receptive prominence or stigma. 

 The reception of the microspores on the terminal region of the 

 carpel or sporophyll is not, however, an exclusive characteristic 

 of the angiosperms, since the same feature is presented by the 

 surviving araucarian conifers. The megasporophylls in this 

 largest and most important group of the seed plants are either 

 folded upwardly upon themselves in such a manner as to inclose 

 the young seeds or ovules, or undergo protective fusions with 

 other similar structures in the same flower. The angiosperms 

 are characterized anatomically throughout by the possession of 

 vessels, these structures being absent only in certain aberrant 

 representatives of the Cactaceae, Crassulaceae, and Trochoden- 

 draceae. In all the exceptional cases mentioned there is clear 

 evidence on comparative anatomical and experimental grounds 

 that vessels were formerly present. The possession of histological 

 structures in the wood known as vessels may accordingly be set 

 down as a primitive feature of organization of the angiosperms. 

 Associated with the vascular structures just indicated, there is a 

 general improvement in the storage devices of the wood which, 

 in an extreme form, leads to the appearance of the herbaceous type. 

 The gametophytes, particularly the male gametophytes, of the 

 angiosperms present a very marked degree of uniformity in the 

 two great divisions of the group. The microspore or pollen grain 

 undergoes part of its germination in the microsporangium and 

 develops normally a tube nucleus and a generative cell. The 



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