﻿2l8 



RELATIONSHIPS. 



borne free on the carpellary leaf, fig. 127, the ovules of Cycas are slightly flattened 

 laterally, and bilobate, being in size and coloration quite plum-like. In the other 

 genera the inclosed position in the cones produces characteristic appression faces 

 011 the relatively smaller ovules, though all are the largest known in seed plants. 



The earlier stages of ovule development, up to that shown in figure 127 A, 

 require but brief mention here, since it is seemingly unlikely that they will ever 

 be observed in the fossil condition, though this is by no means conceived wholly 

 impossible. It is, then, only of present importance to note the fact that the earliest 

 observed stage of megasporangial growth consists in a mass of subepidermal 

 sporogenous cells (as in Ophioglossacese, etc.). Next, the sterile envelope of cells 

 rapidly organizes into a large apical nucellar region and a thick integument pierced 

 by a long micropylar passage. At this stage the sporogenous tissue is deeply 



Fig. 127. — Cycas circinalis. Developing and mature ovules. 



A. Young seed showing an initial stage of the embryo sac. X 3. 



B. Further advanced stage of embryo sac with peripheral endosperm formation previous to growth of archegonia. X 3. 



Lettering of A and B : n, nucellus; e, embryo sac; p. pollen chamber: g. gum canals. 



C. Diagrammatic longitudinal section through nearly mature ovule borne on base, with bilateral symmetry in the plane 



of the carpellary blade. About natural size, a, s, d. respectively the outer fleshy, middle stone, and inner fleshy 

 layers of the seed wall; n, nucellus; e. prothallus or endosperm with archegonia; m. micropylar tube; p, pollen 

 chamber; c. i, k, o, bundle system of the seed walls, of which the central bundle (c) spreading out into the basal 

 system (i) is the primary analogue of the bundle supply of the Cycadeoidean seed ; o, bundle of outer flesh with 

 branch (k) in inner flesh. (Compare with figure 1 28, 5 and 6. ) 



[The preferable interpretation of structure may be that the internal vascular system (i and k) belongs wholly to 

 the nucellus. since in the primitive cycadaceous seeds Stephanospermum. Trigonocarpon, etc., the nucellus with a 

 well-developed bundle system is free from the chalaza upwards.] (A and B from Luersson ; C from Marie C. Slopes. ) 



embedded near the base of the nucellus. Meanwhile the summit of the nucellus 

 assumes a conical form and projects into the micropyle as a cap of firm-walled cells, 

 beneath which is an irregularly conical cavity, the pollen chamber, opening out by 

 a narrow apical passage just beneath the micropyle, and capable of containing a 

 considerable number of polleu grains. Since in Zamia floridana the micropylar 

 tube has a diameter little in excess of a single grain, the pollen is supposed to make 

 its way to the pollen-chamber opening by some form of suction. However this 

 may be, at pollination time the chamber is often packed with pollen grains. 

 While pollen has never been observed inside the integument of any cycadeoidean 



