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PHYSIOLOGY. 



Ovules of Gymnospermia. 



The ovules of the Gymnosperms, Pinaceae and their allies and 

 Cycadaceae, are produced upon open carpels, so that the pollen- 

 grains have direct access to the micropyle (fig. 607, A, a). In 

 Pinus two of these occur at the base of the carpel] ary scale. Each 

 consists of a nucleus (or macrosporange) with only a single inte- 

 gument (fig. 605, A). In this first figure the primary embryo-sac, 

 or " macrospore," is represented in the centre as still very small. 

 Before the pollen-grains fall on the micropyle of the ovule, the 



Pig. 604. Diagrammatic section of an ovule : a, nucleus ; b, embryo-sac ; c, inner coat ; d, outer 

 coat; e, micropyle ; f, chalaza; g, funiculus. 



Fig. 605. Young ovules of Pinus. A. Vertical section at the time when the primary embryo- 

 sac is a small cell in the centre of the nucleus: m, micropyle. B. Section of an 

 older ovule : m, micropyle with two pollen-grains on the apex of the nucleus; e, 

 the primary embryo-sac filled with cellular tissue. Magn. 50 diam. 



embryo-sac becomes filled up, by free-cell formation, with delicate 

 cellular tissue (endosperm-cells), which soon disappear, to be re- 

 placed by a fresh development at a subsequent period. This en- 

 dosperm is the female prothallus. Fig. 605, B, represents a section 

 of an ovule with an embryo-sac (e) filled up in this way, and two 

 pollen-grains which have penetrated into the micropyle (m) pushing 

 their pollen-tubes into the substance of the nucleus. 



In the upper part of the mass of the last formed endosperm (), 

 from five to eight cells are found to expand more than the rest, 

 forming secondary embryo-sacs or corpuscula. These are not formed 

 in the superficial cells of e, but from cells of the second layer, so 

 that each is separated from the membrane of the primary embryo- 

 sac by one cell (fig. 606, A) These corpuscula, as they were called 

 by Robert Brown, their discoverer, are very much like the arche- 

 gonia in the internal prothallium structure of Selaginella. After 

 a time the secondary embryo-sacs divide into an upper or neck-cell, 

 and a lower or central cell, egg or oosphere. The neck-cell speedily 





