To Face Page 136. 



EXPLANATION OF PLATE. 



Fig. 



(pr.) developed from the spore (sp.) to which it : 



still 



ule of an Angiosperm, Two 

 ntents from the rest of the 

 ■. the archesporium. 



-Fern-prothallium {pr.) quite freed from the spore, and bearing the young 

 asexual plant (pi.) a. an archegonium, the oosphere of which has not 

 developed {longitudinal section). 



-Young asexual plant of Salvinia {pi.) borne on the prothallium (//-}, 

 which has not become quite freed from the macrospore (s^,). 



-Longitudinal section through a macrospore of Isoites filled with the 

 prothallium which bears an archegonium (ar.) at the apex. 



-Microspore of Isoctes, showing formation of antherozooids (an.) ; pr., 

 prothallium-cell. 



-Microspore (Pol ten -grain) of Finns sylvestns : pr.. prothallium-cell; an., 

 antheridial cell, which will grow out as the pollen-tube. 



-Pollen-grain of an Angiosperm ; it., nucleus ; pr., prothallium-cell, 



-Ditto, in germination, showing production of polien-tube 



-Longitudinal section of ovule of a Gymnosperm, showing contents of 

 macrospore (embryo-sac), method of fertilisation and development of 

 embryo; /.integument; nii., nucellus ; /r., prothallium (endosperm) 

 contained in the embryo-sac ; os., oospore, sho^ving primary cell 

 divisions as result of fertilisation ; e , embryo developed from the 

 other oospore with which it is seen to be still connected ; p., pollen- 

 grain germinating in the micropyle ; p.t,, pollen-tube which has 

 fertilised the oospheres, 



-Longitudinal section through the young ( 

 cells are distinguished by size and c 

 nucellus ; t. is the primary tapetal cell, 



-Later stage of lo. The archesporium has developed into a row of cells, 

 the lowest of which [iita.) will become the embryo-sac. 



-Still later stage of same, showing the ovule, consisting of nucellus (nii) and 

 outer (o i ) and inner (/./ ) integuments, and also the contents of the 

 embryo-sac {>mi.) : o., oosphere ; syn., synergidse ; tint., antipodal cells. 



— Longitudinal section of nucellus of Rosii liviihi, showing the sporogenous 

 tissue distinguished by great elongation of the cells, large nuclei, 

 and conspicuous cell-contents. Tabular tapeial cells, alsp distinguished 

 by nuclei and contents, are seen above the sporogenous tissue. 



-Sporogenous tissue of a Cycad surrounded by a layer of tapetal cells (/.). 



—Longitudinal section of ovary of Angiosperm at the time of fertilisation ; ov.. 

 ovary-cavity ,o.i.. outer integument of ovule; ( /.inner ditto; nu., nucellus 

 containing the mature embryo-sac in which are seen o.. the oosphere, 

 5y;i., synergidae, ((/</., antipodal cells ; p.t.. pollen-tube making its way 

 from the grain which germinated on the stigma, down the style and 

 through the ovary-cavity and micropyle to the apex of the nucellus. 



-Seed of an Angiosperm containing the embryo (e.) surrounded by endosperm 

 (emt). 



—Longitudinal section of nucellus of Casnarina showing development of 

 sporogenous tissue (dark-celled). After Treub. 



—Longitudinal section of sporogenous tissue 

 large macrospores {mu.). A tracheid is in 



-Longitudinal section of ovule after passage of pollen-tube ; ov., ovary-cavity ; 

 o.i., i.i., outer and inner integuments; s. ma., sterile macrospores ; 

 f. ma., fertile macrospore containing sexual apparatus; M . chalaza ; 

 p.t., pollen-tube passmg down the tissue of the placenta 'to the chalaza. 

 where it branches, the main branch passing up the tail of a sterile 

 macrospore (nnt indicated) and becoming attached to the fertile 

 macrospore. The constriction of the pollen-tube is shown. 



