. A textbook of botany for colleges and universities ... Botany. SPERMATOPHYTES 23s Sfii< antipodal region are multinucleate, and those of the micropylar region become so (two to five-nucleate). Since there is no uninucleate cell in the micropylar tissue, there is no archegonium initial, and hence no archegonium. Instead, several of the multinucleate cells develop what have been called prothallial tubes, which penetrate the overlying nucellar tissue, and into them the nuclei pass, each nucleus being a potential egg nucleus. It is at this point that the archegonium dis- appears ; which seems
. A textbook of botany for colleges and universities ... Botany. SPERMATOPHYTES 23s Sfii< antipodal region are multinucleate, and those of the micropylar region become so (two to five-nucleate). Since there is no uninucleate cell in the micropylar tissue, there is no archegonium initial, and hence no archegonium. Instead, several of the multinucleate cells develop what have been called prothallial tubes, which penetrate the overlying nucellar tissue, and into them the nuclei pass, each nucleus being a potential egg nucleus. It is at this point that the archegonium dis- appears ; which seems to be associated with the fact that the egg nuclei are differen- tiated before wall formation in the en- dosperm has been completed. —In this genus there is free nuclear division as before, but wall for- mation does not oc- cur, so that the em- bryo sac at the time of fertilization con- tains only free nuclei (fig- 531)1 and each one of these free nuclei is a potential egg nucleus. This is the general angiosperm condition. Below the antipodal end of the sac a remarkable nutritive (glandular) tissue is developed. Male gametophyte. — The male gametophyte of Gnetales is known only in Ephedra. In its shedding condition the pollen grain of Ephedra contains two persistent vegetative cells, and conspicuous stalk, body, and tube nuclei (fig. 532). In this condition the pollen grain rests on the exposed archegonium necks, and before the pollen tube is formed the body nucleus divides into two equal male nuclei (fig- 533)- Fertilization. — The phenomena of fertilization vary with the struc- ture of the female gametophyte. In Ephedra the pollen tube breaks through the long and massive neck of the archegonium (fig. 529); in Tumhoa it comes into contact with the prothallial tubes that are pene-. 532 Figs. 532, 533. — Male gametophyte of Ephedra: 532, pollen grain in shedding stage, containing two vegetative nuclei (below), stalk {sin) and body (Jbn) nuclei, and tube nucleus
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