5H 



PHANEROGAMS. 



tarpeae their capacity for transport is increased by the vesicular hollow protrusions 

 of the extine, as represented in Fig. 351, 7V, V, hi. [The pollen-sacs (microspo- 

 rangia) of the Coniferae resemble the sporangia of the Vascular Cryptogams in 

 the mode of their development. A section through the pollen-sac of one of the 

 Cupressineae, for example, shows that it resembles a sporangium of Lycopodium : 



Fk;. j^o.—Adtes pectinata ; A a male flower, b the delicate bud-scales forming a perianth, a the stamens ; B a pollen-grain 

 (after Schacht), e its extine, forming the two large vesicular protrusions bl. 



in the centre is a group of sporogenous cells surrounded by a layer of flattened 

 tabular cells, the tapetum, and externally is the wall of the sporangium. From 

 Goebel's^ researches it appears that the archesporium, in Biota orientalis at any 

 rate, is a hypodermal cell, the terminal cell of one of the axial rows of cells of 



Fig. 351.— y^ pollen-grain of Thuja orientalis before its escape from the pollen-sac, / fresh, //, /// a''ter lying in 

 water, the extine e having been stripped off by the swelling of the intine i; B pollen-grain o( Piiius Pinaster before its 

 escape, e the extine with its vesicular protuberances bl. 



which the young pollen-sac consists. Possibly the archesporium consists in this 

 case, as in Lycopodium, of a transverse row of such hypodermal cells. The cells 

 of the tapetum are derived here, as in Selaginella, partly from the archesporium 

 and paitly from the tissue of the wall of the pollen-sac. In Pinus the pollen-sac 



^ [Goebel, Vergl. Entwick. d. Sporangien, Bot. Zeitg., 1881.] 



