136 ^ NATURAL SCIENCE. .^p^.^.. 



very modified staminal leaf. The sporogenous tissue is the product 

 of an archesporium and is surrounded by tapetal cells. The micro- 

 spores or pollen-grains are formed by division of the mother-cells into 

 four daughter-cells, but the development of the macrospores is of 

 greater interest. 



Let us first consider the Gymnosperms, as it was here that the 

 relation to the Vascular Cryptogams was first pointed out by 

 Hofmeister. The niicelliis, or group of cells composing the sporangium, 

 is surrounded by a single integument, except at the apex, where an 

 open passage is left, the micropyle. The macrospore or embryo-sac is 

 produced from a hypodermal archesporium, which is covered above 

 by tapetal cells as in Isoetes. The archesporium may divide by a few 

 shiny transverse walls into a row of cells, the lowest one of which is 

 the macrospore, as for instance in the Larch (Larix). In the Cypress 

 (Cupyessns) and Callitrisy the archesporium may, however, divide 

 further, forming a group of cells, as happens also in the Cycadeae. 

 A sporogenous tissue is thus produced ; all the cells are, however, 

 sterile except the one which produces the embryo-sac, and are 

 replaced, as in Isoetes, by the growing macrospore. There is, more- 

 over, a difference from the Vascular Crytogams, in that the mother-cell 

 does not divide by a repeated bipartition into four daughter-cells or 

 macrospores, but elongates and divides by superposed transverse walls. 

 In the Cycads, the cell- wall of the embryo-sac thickens and splits into 

 two layers, the outermost of which is cuticularised like the membrane 

 of a macrospore, which will be set free from the sporangium. The 

 embryo-sac remains in the nucellus, almost the whole of which it 

 occupies, and the nucellus remains attached to the carpel, and 

 thereby to the parent plant. The macrospore germinates as in 

 Isoetes. The nucleus, by repeated bipartition, gives rise to a number 

 of free nuclei round which cells are formed ; these grow and 

 divide in such a way as to fill the macrospore with the tissue of 

 the prothallium. Archegonia are formed at the apex from single 

 superficial cells of the prothallium in the same manner as in the 

 Vascular Cryptogams. 



It is obvious, however, that fertilisation cannot be affected in the 

 same way. In Vascular Cryptogams the prothallium, either free or 

 still contained in the ruptured spore is found on damp ground 

 or in mud at the bottom of water. But free-swimming antherozooids 

 would be incapable of penetrating to the oosphere borne high and dry 

 on the cone of a Fir-tree. Hence the mechanism is different. The 

 pollen-grains when set free from the pollen-sac, contain an indication of 

 a prothallium in one or two cells cut off from the rest of the contents. 

 The light grains, produced in large quantities, are carried by the 

 wind. Some find their way to the carpel and the micropyle of the 

 ovule, and pass through it to the apex of the nucellus. The anthe- 

 ridial cell grows out as a short tube into the tissue of the nucellus, 

 where it remains stationary for a time. Then, when the oosphere is 



