ANGIOSPEBMiE 181 



DIVISION B.—ANGIOSPEBMM. 



In this division the evokition of the flower reaches its highest 

 point ; the micro- and niacrosporophylls are usually found in 

 the same flower ; the former are, as a rule, much modihed in 

 shape, and their foliar character [is at first sight diflicult to 

 grasp ; they form the structures described ah-eady as stamens, 

 each consistmg normally of a filament and an anther, the 

 latter bearing the microsporangia or pollen-sacs. The macro- 

 sporophylls are carpels, and are either distinct or coherent to- 

 gether. In either case the ovules or macrosporangia are con- 

 tained in a closed cavity, or ovary, formed by the cohesion in 

 various ways of the carpellary walls. Each ovary is crowned 

 by a stigma, which is often separated from the ovary b}^ a 

 filiform structure called the style. 



The pollen grain, instead of entering the micropyle of the 

 o\-ule and germuiating there, faUs upon the stigma and there 

 develops its gametophyte, which penetrates the tissue of the style 

 in the form of the pollen tube and makes its way into the cavity 

 of the ovar}^ ultimately reaching the ovules. It generally enters 

 the latter at the micropyle and so makes its way to the gameto- 

 phyte of the macrospore or embr^'o sac, which is situated just 

 below the micropyle. In a few cases it finds its waj^ to the 

 female cell by burrowing through the tissue of the ovule and of 

 the gametophyte from the chalazal end. This, however, is 

 exceptional, and only occurs in a few families. 



The gametophyte produced by the macrospore is always com- 

 pletely internal or endosporous, and is very much reduced. It 

 bears the female cell or oosphere at its micropylar end, and tliis 

 cell is not enclosed in an archegonium {fi^. 935). 



Histologically the group shows an advance upon the structure 

 of the Gymnosperms in the greater variety of the elements com- 

 posing the wood and bast. The wood contains true vessels, which 

 show the varieties of thickening already described. The sieve 

 tubes always have companion cells. The development of the 

 spores takes place in the way already described. In a few cases 

 modifications of the process are found. In Asclepias and Zostera 

 the mother cells of the poUen do not form four special mother 

 ceUs, but each becomes a microspore. 



