G YMNOSPERMAE. — CYCADEA E. 3 I 7 



infolded deeply in the dry pollen-grain, which therefore looks kidney-shaped in a 

 transverse section ; but it resumes its spherical form with the absorpdon of water 

 which precedes the development of the pollen-tube. 



The carpellary leaves are closely crowded on the axis of the female flower in 

 spirals or in apparent whorls. Those of Cycas have been already described ; in 

 Zajnia, Encephalartos, Alacrozamia, and Ceratozamia the carpels are much smaller and 

 bear each of them only two macrosporangia or ovules, one 

 right and one left, on the upper peltate portion which is 

 supported on a slender basal part or stalk. The macro- 

 sporangium (ovule) is always orthotropous and consists of 

 a large nucellus and a thick and solid integument, the inner 

 tissue of which is traversed by numerous vascular bundles. 

 The microjjyle is a slender tube formed by the drawing 

 together and elongation of the margin of the integument. 

 The group of cells which produces the spores is at the base 

 of the nucellus ; and we may presume from analogy that 

 these cells proceeded from a hypodcnnal archesporiuni which 

 was probably unicellular ; but, as in Selaginella, only one cell 

 of the sporogenous group is further developed. This cell 

 soon becomes distinguished from the rest by its size and 

 divides into three cells, the lowest of which usually becomes 

 the macrospore or embryo-sac, and supplants the other fig. 248. ceratozamia longc 



, ^ , . , J 1-^ ■^''^'''- ■■' Po'lengrain before ger- 



two. The cell-wall of the macrospore thickens and splits mination with the three-ceiied body 



1 ^u 4^ <. f 1,- U • ;-• 1 • J -^^ '^ pollen-grain germinating; e 



into two layers, the outermost 01 which is cuticularised, the exine;/^- the poiien-tube formed 



.... from the inline, j" the inclosed cel- 



just like the membrane 01 a macrospore which is to be luiarbody. After juranyi. 

 released from the sporangium. 



The development of the macrosporangia is not so well known in the Cycadeae 

 as in the Coniferae, though Treub' has recently published some important facts in 

 the history of the macrosporangium of Ceralozamia loTigifolia. The rudiments of the 

 sporogenous cells are visible as a group of cells sunk in the tissue of a lateral lobe of 

 the carpellary leaf before any external differentiation of the macrosporangium has 

 taken place, in other words, the macrosporangium in its early stages resembles that 

 of plants like Ophwgl'>ssum. The nucellus is produced by the luxuriant growth of 

 the cells which lie above the young sporogenous group, and which in a sporangium 

 of Ophioglosswn become simply the wall of the sporangium, and the integument 

 grows up like a circular wall round the nucellus. If then we compare the history of 

 the development of the macrosporangium of the Cycadeae with that of the sporangia 

 of the Vascular Cryptogams, it appears that the nucellus is essentially only a 

 luxuriant growth of the outer wall of the macrosporangium^, while the integument is 

 a new formation ; the macrospores are not formed by division of a mother-cell into 

 four daughter-cells as in heterosporous Vascular Cryptogams. 



The macrospore as it grows exercises a destructive effect on the surrounding 

 cells, like the macrospore o^ Isoetes for example, and becomes filled, as in Isochs, wiih 



' Recfi. sur les Cycadees (Ann. d. jaid. Bot. de Buitenzoig, II, 1S81. p. \i ol ihe repiint. 

 - Gobel, Bot. Ztg. 1881. 



