478 PART III. THE CLASSIFICATION OF PLANTS. 



The female prothallium is a mass of parenchymatous tissue, 

 which does not, as a rule, escape to any extent from the spore, 

 and which, in consequence of the exclusion of light, is destitute 

 of chlorophyll ; the only exception to this rule is offered by the 

 Cycadaceae where, if the female organ is not fertilised, the pro- 

 thallium, resuming its growth, protrudes through the micro pyle 

 and turns green in the light. 



The female organ is an archegonium, and is developed from a 

 single superficial cell of the female prothallium at its micropylar 

 end. The mother-cell generally divides transversely into two ; 

 an upper, the neck-cell ; a lower, the central cell : the neck-cell 

 usually divides, by two vertical walls, into four cells, which form 

 the neck ; the central cell grows, and divides transversely at its 

 upper end so as to cut off a small cell, the canal-cell, which lies 

 in the canal formed by the separation of the neck-cells, and a 

 large cell which is the female cell or oosphere (Figs. 301, 302). 



The most striking deviations from the development of the archegonium, as 

 described above, are the following : the mother-cell does not divide but becomes 

 the central cell of a necldess archegonium, either with a cell-wall (Welwitschia) 

 or without a cell-wall (Gnetum); the neck-cell remains undivided (Tsuga cana- 

 densis) ; the neck-cell divides only once, longitudinally, so that the neck con- 

 sists of two cells (e.g. Cycadacese, Ginkgo) ; by a third vertical division of the 

 neck-cell, the neck comes to consist of eight cells, all in one plane (some 

 Cupressinese) ; by one or more transverse divisions, the neck consists of two 

 or more tiers of cells (e.g. Pinus Pinaster with four cells in each tier ; Picea 

 excelsa with eight cells in each tier). 



The number of archegonia developed on the female prothallium 

 varies from a small number (3-5) in the Abietineae, to a large 

 number (20-60) in Welwitschia and Gnetum. The archegonia 

 are either scattered (Abietineos), or in a group (Cupressineaa) : 

 when scattered, each central cell is surrounded by a layer of small 

 cells belonging to the prothallium ; when in a group, the central 

 cells are in actual contact and have a common investment of 

 small-celled tissue. 



In Welwitschia, the unicellular archegonia grow out into 

 filaments, at the upper end, which make their way through the 

 tissue of the nucellus toward the micropyle. 



The female cell or oosphere is a relatively large nucleated cell, 

 the protoplasm of which is so highly vacuolated that it presents a 

 frothy appearance. 



Fertilisation. When the microspore has reached the apex of 



