1904 | WYLIE: ELODEA CANADENSIS 9 
In the second division the spindles commonly lie parallel, in 
which case the four spores of the tetrad develop in one plane; 
but the spindles may be rotated so that their planes intersect at 
right angles, resulting in a correspondingly different grouping 
of the pollen grains. Following the second division, the four 
microspores are organized in the usual manner. The young 
spores lie for a time within the wall of the mother-cell (fig. 50), 
but the small tetrads soon appear naked (fig. 57) and enter 
upon a period of rapid enlargement. The four members of the 
tetrad do not fall apart, but remain attached and are ultimately 
shed from the sporangium still firmly joined together. This 
union is so intimate at maturity that violent shaking in a closed 
vessel partly filled with water seldom breaks them apart. 
Though borne by one of the most specialized of submersed 
aquatics, a plant entirely devoid-of cutinized walls in all its vege- 
tative parts, the microspores of Elodea exhibit a strongly cuti- 
nized exine and a well-developed intine. In this connection it is 
interesting to note that in Naias and Zannichellia, also submersed 
aquatics, Campbell (1) finds no exine developed. Strasburger 
(16) reports for Ceratophyllum, which has a similar habitat, a 
pollen grain without intine and with a thin smooth exine which 
is cutinized, but not nearly so strongly as in air blooming plants. 
In Elodea the exine closely resembles such coats borne in normal 
aerial sporangia, and is beset with multitudes of spines, which 
play an important part in the process of pollination, as we shall 
see later. These spines (fig. 67) are cylindrical, with conical 
points barbed at the base and each bearing at its tip a tiny disk. 
The intine possesses in numerous instances peculiar thicken- 
ings reaching into the cavity of the spore. These ingrowths 
may be merely papillae, though they are often rod-like, or even 
membranous, forming trabeculae extending well across the pol- 
len grain. These protrusions are integrated with the intine at 
their junctions, and appear to be perfectly uniform in character 
and substance with it. 
The microspore nucleus divides long before the pollen grain 
has attained its full size ( fig. 52), and at a time when the spore 
wall is not yet differentiated into two layers. This first division 
