COMMON CYCADS crn eae 
and increase its thickness by the development of their 
cambium, and by the formation of new bundles in the 
cortical meristem. The _ sporophylls 
which bear microspores and megaspores % O 4 
form more or less distinct cones (strobili) . 
but occur on separate plants (dioecious). i 
489. The common greenhouse Cycad “== | 
(Cycas revoluta) produces elongated, 
compact cones of microsporophylls, 20 sage, 1S —Lreae 
to 30 centimeters long and 5 to 6 centi- gporophylls ind mi- 
meters thick. Each sporophyll bears on 
its lower surface numerous small scattered microspor- 
angia containing microspores, constituting the so-called 
“‘pollen.”’ These microspores fall out, and on germi- 
nation produce a small one- or two-celled gametophyte, 
and a tubular antherid containing two spirally many- 
ciliated sperms (about 0.2 millimeter in diameter). The 
megasporophylls constitute a loose terminal cone on the 
main axis of the tree. Each sporophyll bears several 
laterally placed megasporangia each of which has become 
covered with an indusium-like structure (integument). 
Within the body of the sporangium (now known as the 
ovule) a megaspore develops, but this at maturity does 
not fall out but remains surrounded by nutrient tissue. 
Here it germinates and develops a solid, many-celled 
spheroidal gametophyte, and at its summit forms sev- 
eral deeply sunken archegones, in which the eggs are of 
remarkably large size (2 to 3 millimeters). 
490. Fertilization of the egg takes place as follows: 
The microspore is carried by the wind or other means to 
the opening (micropyle) at the summit of the ovule 
integument; there it germinates, the tubular antherid 
penetrating the adjacent tissues; the sperms escape by the 
rupture of the tube, and swim through the intervening 
18 
