SEED-PLANTS. 30I 



spermatozoid, which with the help of water is able lo make its way through the open 

 neck of the archegonium to the oosphere ; in the Seed-plants, where the oosphere 

 is enclosed in the embryo-sac and nucellus, in the Angiosperms in an ovary also, 

 such a mode of transmitting the fertilising substance would be ineffectual ; the 

 pollen-grains are themselves therefore conveyed to the vicinity of the oosphere by 

 external agencies, by the wind, by mechanical contrivances in the flowers, most often 

 by insects ; there each grain germinates like a spore and sends out the pollen-tube, 

 which growing through the tissue of the female sporophyll at length reaches the 

 embryo-sac and transmits the fertilising matter to the oosphere. How the trans- 

 mission is effected through the closed wall of the pollen-tube is unknown; but the 

 phenomena of fertilisation within the oosphere, as at present known, are the same as 

 the analogous processes in the Vascular Cryptogams. 



The microsporangia also of the Seed-plants have a name of their own and are 

 termed pollen-sacs. Their structure and origin and the formation of the microspores 

 or pollen-grains by division of ihe spore-mother-cells (pollen-mother-cells, sporocytes) 

 into four parts agrees in the minutest particulars with the details given above in the 

 case of the sporangia of the Vascular Cryptogams. On the other hand the sporo- 

 phyll or axis, which bears the microsporangia and is termed the stamen, is often of 

 peculiar construction ; yet the sporophylls of the Cycadeae and Coniferae are in 

 shape and position just like those of many Vascular Cryptogams, and are more or 

 less modified foliage-leaves which usually bear the microsporangia or pollen-sacs on 

 their under surface. 



The important point which results from these considerations is that the seed- 

 bearing plant with its pollen-grains and embryo-sacs is the equivalent of the 

 spore-producing generation (sporophore, sporophyte) of the heterosporous Vascular 

 Cryptogams. But as in the Vascular Cryptogams the sexual differentiation appears 

 first in homosporous Ferns and Equisetaceae in the prothallium alone, and then in the 

 heterosporous Filicineae and Lycopodineae at an earlier stage, namely, in the spores, 

 so in the Seed-plants it is carried still further back, and is manifested not only in the 

 formation of the macrospore or embryo-sac and microspore or pollen-grain, but also 

 in the difference between macrosporangium or ovule and microsporangium or pollen- 

 sac, and even before this in the distinction between male and female flowers and 

 between male and female plants (dioecism). 



The oospore of the Seed-plants does not as a rule develope directly into the 

 embryo, but a pro-embryo is formed, which by growth first of all in the direction of 

 the base of the embryo-sac and by division produces the suspensor which we have 

 already seen in the Selaginelleae, and the embryo is developed subsequently from a 

 usually roundish mass of tissue at the apex of the suspensor. The embryo has usually 

 advanced so far in its development before the seed is fully ripe, that the first leaves, 

 and primary stem-axis, and the first root can be easily distinguished ; it is only in 

 parasitic plants which have no chlorophyll and in saprophytes that the embryo 

 continues in a rudimentary condition without perceptible external differentiation till 

 the time that the seeds are dispersed ; in Phanerogams containing chlorophyll it is 

 often of very considerable size and the external differentiation of its parts very far 

 advanced, as for instance in Pinus, Zea, Aesculus, Quenus, Fagus, Phaseolus, and 

 others. Apart from the curvatures which often occur in the embryo, the apex of its 



