3i6 



FO UR TH GR UP. - SEED-PLA NTS. 



fully known in Zinnia /)iiirua/a\ but even here there are points not yet cleared up. 

 A lobe is formed on the right and left side at the base of the young staminal leaf. 

 and on these lateral expansions, which are to be regarded perhaps as rudimentary 

 pinnae, the placentas appear in the form of hemispherical protuberances, six on 



each lobe. Two microsporangia or 

 pollen-sacs are formed on each placental 

 protuberance essentially in the same way 

 as in the Marat tiaceae, and here as in 

 them there is without doubt a unicellular 

 archesporium. In a later stage of their 

 development the microsporangia consist 

 of a wall of several layers of cells and an 

 inner group of larger cells filled with 

 dense protoplasm, the pollen-mother- 

 cells, which are invested by a double 

 layer of narrow thin-walled tapetal cells. 

 The pollen-mother-cells divide first into 

 two and then into four daughter-cells, as 

 in most Monocotyledons. The micro- 

 spores or pollen-grains of Ceratozamia 

 when released from the sporangium are 

 unicellular and spherical (Fig. 248); but 

 as they increase in size their contents 

 which are enclosed in an exine and an 

 intine divide into two cells, a large and 

 a small one, each with a nucleus. The 

 small cell lying on one side in contact 

 with the intine of the grain bulges out on 

 the other side and grows in the form of 

 a papilla into the large cell; then the 

 smaller cell divides transversely, parallel, 

 that is, to the first division of the grain, 

 and a second division sometimes follows ; 

 in this way a two-celled or three-celled 

 body is produced resting on the intine 

 and projecting into the cavity of the larger cell, as in the Abietineae, from which 

 however Ceratozamia differs, because the large cell produced in it by the first division 

 of the pollen-grain developes into the pollen-tube as in the Cupressineae, while the 

 group of small cells, the rudimentary prothallium, remains inactive. In Cycas 

 Riimphii, Encephalartos, and Zarnia, according to De Bary, the pollen-grain divides 

 in the same way into a large and a small cell, and the latter divides again, and here 

 also it is the large cell which developes into the pollen-tube. The spot where the 

 protuberant intine breaks through the exine is diametrically opposite to the small 

 cellular body, the prothallium of the grain; at this spot the exine is thinner and 



Fig. 247. Zntnia muricata. A a male flower of tlie natural 

 size. B transverse section of the same. C a stamen with the pollen- 

 sacs X and the peltate scale s to which they are attached seen from 

 beneath. D the upper part of a female flower of the natural size. 

 H transverse section oi D ; s the peltate scale which bears the 

 ovules sk. F ripe seed in longitudinal section ; e the prothallium 

 (endosperm), c the cotyledons, at x the rolled up suspensor. After 



Treub, Recherches sur Its Cycadees (Annales dxi jardin Bot. de Kuitenzorg, Vol. II, pp. =;2, 53). 



