364 PART IV. — VEGETABLE TAXONOMY. 



which we have already studied as the flower. Moreover, the 

 ovules and pollen-sacs of seed-plants are homologous respect- 

 ively with the macrosporangia and microsporangia of the higher 

 Pteridophyta, and the embryo-sac in the ovule corresponds with 

 the macrospore, and the pollen-grain with the microspore. 



In the lower Pteridophyta, such as Ferns and Equisetums, we 

 found that the prothallium or sexual generation attains a very 

 considerable development, and continues some time as an inde- 

 pendent plant, but in the higher members of the Series, as Sal- 

 vinia, IsoStes, etc., it becomes progressively of less and less 

 importance, in some cases scarcely emerging from the coats of 

 the spore. In Seed-plants the prothallium is still represented, 

 but its degradation is carried a step farther. It consists of a 

 few cells or a tissue formed in the embryo-sac, and it never 

 bursts through the walls of the latter, but either remains as a 

 part of the seed, or is afterwards absorbed by the forming 

 embryo. The seed is, in fact, a macrospore which has, while 

 still in contact with the parent plant and nourished by it, germi- 

 nated and produced an internal prothallium, and a germ-cell or 

 oosphere, which, after fertilization, has developed into an embryo, 

 and only after these processes have been accomplished, has 

 become free. 



In like manner, the pollen-grains are microspores somewhat 

 modified in accordance with the changes in the development of 

 the macrospore. They are formed in the pollen-sac in the same 

 way that microspores are in microsporangia, but the male pro- 

 thallium which they produce in germinating, is, if possible, still 

 more rudimentary or aborted altogether, and no motile anthero- 

 zoids are produced. Instead, the entire pollen-grain, as we have 

 found, is conveyed by the wind, by insects, or by some other 

 agency either to the ovules direct, as in the Pines and their rela- 

 tives, or to the stigma, which is a portion of a leaf or whorl of 

 adnate leaves enclosing the ovules, as in the higher flowering- 

 plants, and it there germinates and forms a pollen-tube which 

 penetrates to the ovule, and through the micropyle of the latter 

 to the surface of the embryo-sac, as was explained in Part I. 



Compared with the preceding Series, the various organs of 

 vegetation are more complex and better developed, the modifica- 

 tions of form and structure which organs of the same name assume 



