SPERMATOPHYTA 429 
Sporophyte. The sporophyte is a complicated plant with roots, 
one or more stems, leaves, and _ strobili (aggregations of sporo- 
phylls) known as flowers. 
The wood is highly specialized in that rows of tracheids have 
become transformed into vessels, while other tracheids are special- 
ized as wood fibers. The vessels are greatly superior to tracheids 
for the conduction of water, and in most angiosperms the function 

Fig. 485. Development of female prothallus in an orchid 
First figure, apical megaspores are disorganizing, while basal megaspore has 
enlarged and its nucleus has divided to form two nuclei. Second figure; older 
stage, showing formation of vacuole. Third figure, nucleus at each end has 
divided, resulting in four nuclei in the prothallus. Fourth figure, telophase of 
division of four nuclei to form eight. Fifth figure, six nuclei cut off by walls; in 
the base there are three antipodal cells; at the tip are a large egg cell and two 
smaller synergids; between the groups of cells are two free polar nuclei. Sixth 
figure, fusion of the polar nuclei. (Redrawn after Brown and Sharp) 
of the tracheids is largely that of mechanical support. The develop- 
ment of an efficient water-conducting system composed of vessels 
has enabled angiosperms to produce a great display of mesophytic 
foliage. The improvement in the conducting system has there- 
fore resulted in a great development of the assimilating system. 
Another great advance shown by angiosperms is the develop- 
ment of complicated structures which facilitate insect pollination. 
Sporophylls. There are two kinds of sporophylls, megasporo- 
phylls and microsporophylls. When there is only one megasporo- 
phyll the edges are joined to form an ovary with a single cavity, 
