EVOLUTIONARY HISTORY OF PLANTS 259 
aquatic plant sometimes known as a water-fern (Fig. 176), 
produce two kinds of spores, the large ones known as 
megaspores, and the small ones known as microspores 
(Fig. 177). Both kinds produce microscopic prothallia, 
those of the former bearing only archegonia, those of the 
latter only antheridia. From the 
prothallia of the macrospores a 
plant (non-sexual generation) of 
considerable complexity of struc- 
ture is formed. 
336. Parts of the Flower which 
correspond to Spores.—In seed-  py6 477, Two Indusia 
plants the spore-formation of of Salvinia. 
cryptogams is represented, though mé, microspores; ma, mega- 
in a way not at all evident without oe 
careful explanation. The carpel is the macrospore-producing 
leaf or megasporophyll, and the stamen is the microspore- 
producing leaf or méerosporophyll. Pines and other gym- 
nosperms produce a large cell (the embryo sac) in the ovule 
(Fig. 178) which corresponds to the macrospore, and a pol- 
len grain which represents the microspore. In its devel- 
opment the macrospore produces an endosperm or small 
cellular prothallium, concealed in the ovule. The micro- 
spore contains vestiges of a minute prothallium. 
In the angiosperms the megaspore and its prothallium 
are not less developed, but the microspore or pollen grain 
has lost all traces of a prothallium and is merely an an- 
theridium which contains a generative cell This is most 
easily seen in the pollen grain, but sometimes it is plainly 
visible in the pollen tube (Fig. 123). 
1 See Bergen and Davis’ Principles of Botany, Ginn & Company, Chapter 
XXVIII, especially pp. 379-387. 
