STRUCTURE OF MICROSPORE 371 



B, a single microspore, showing bladder-like processes of outer coat, 

 and contents divided into small prothallial cell (a) and large cell (b), 

 from which the pollen-tube arises. 



C, diagrammatic vertical section of female cone, showing axis with 

 female sporophylls (sp. ph. 9 ) bearing megasporangia (nig. spg), each of 

 which contains a single megaspore (mg. sp) : per, the scale-like perianth 

 leaves. 



D, diagrammatic vertical section of a megasporangium, showing 

 cellular coat (t), and nucellus (ncl), micropyle (mpy), and megaspore 

 (mg. sp) : the latter contains the prothallus (prtli) in which are two 

 ovaries, that to the left showing a large ovum (ov) and neck-cells, while 

 that to the right has given rise to an embryo (emU) which is in the 

 phyllula stage, and has sunk into the tissue of the prothallus by the 

 elongation of the long suspensor (spsr). 



A microspore (mi. sp) is seen in the micropyle sending off a pollen- 

 tube (/. /), the end of which is applied to the necks of the two ovaries. 



E, diagrammatic vertical section of a seed, showing coat (t), micro- 

 pyle (mpy), and endosperm (end], in which is embedded an embryo in 

 the phyllula stage, consisting of stem-rudiment (st), cotyledons (ct), and 

 root (r). 



(A and B, altered from Strasburger ; D and E, altered from Sachs.) 



In the- male cone (A) the sporophylls (stamens, sp.ph.$) 

 are more or less leaf-like structures, each bearing on its 

 under or proximal side two or more microsporangia (pollen- 

 sacs, mi. spg). The mother-cells of these divide each into 

 four microspcres (pollen-grains), which are liberated by the 

 rupture of the microsporangia in immense quantities. The 

 microspore (B) is at first an ordinary cell consisting of proto- 

 plasm with a nucleus and a double cell-wall, but upon being 

 liberated the protoplasm divides, as in Selaginella, into two 

 cells, a small one (a) the vestige of the male prothallus, and 

 a large one (b) which does not develop sperms, but under 

 favourable circumstances undergoes changes which will be 

 described presently. 



In the female cone (c) each sporophyll (carpel, sp. ph. $ ) 

 bears on its upper or distal side two megasporangia (so-called 

 ovules, mg. spg} the structure of which is peculiar. Each 

 consists of a solid mass of small cells called the micellus(>, ncl\ 

 attached by its proximal end to the sporophyll, and sur- 



B B 2 



