206 



INTRODUCTION TO CYTOLOGY 



up into a number of fragments. These later join to form a cilia-bearing 

 band which elongates spirally in close union with the nucleus to form 

 the spermatozoid (Sharp, 1914). 



Gymnosperms. — In all seed plants, as well as in certain groups of 

 pteridophytes, spores of two distinct kinds are produced in quartets: 

 microspores and megaspores.'^^ Such plants are said to be heterosporous, 

 while those with only one kind of spore quartet are homosporous. In the 

 gymnosperms the microspore quartets are developed in a manner essen- 

 tially like that of the quartets of homosporous ferns, i.e., by the subdivi- 

 sion of isodiametric sporocytes. The sporocyte which develops the 







Fig. 123. — OSgenesis in Dioon edule. A, young archegonium. B, egg nucleus below 

 and ventral canal nucleus above; two conspicuous neck cells. C, protrusions of the egg 

 cytoplasm (at right) extending as haustoria through pores in the egg membrane into the 

 surrounding jacket cells (at left). {After Chamberlain, 1906.) 



megaspore quartet in the ovule is commonly elongated, and it divides in 

 such a manner that the spores form a row. The outer three spores 

 ordinarily degenerate, while the innermost one develops into a female 

 gametophyte with archegonia. 



The eggs of gymnosperms^^ are very large cells, the nucleus alone in 

 certain cycads being as much as 0.5 millimeter in diameter. The egg is 

 borne in an imbedded archegonium, which has an evanescent ventral 

 canal cell or nucleus (Fig. 123). The egg cytoplasm is in close communi- 

 cation with surrounding nutritive cells of the gametophyte through pits 

 in the wall. In most cases these pits are said to be closed by a membrane, 

 the only direct connection being through minute plasmodesms. In 

 Dioon, however, Chamberlain has shown that the membrane at first 



^^ These standard terms imply a size difference, but the essential difference is one 

 of sexual tendency; androspore and gijnospore would be more appropriate. "Mega- 

 spores" are sometimes smaller than "microspores." 



11 Goroschankin (1883), Ikeno (1898), Chamberlain (1899, 1906, 1910, 1912, 1916), 

 I. Smith (1904), Ferguson (1904), Stopes and Fujii (1906), Eames (1913), P. Sedgwick 

 (1924), Lawson (1907, 1909, 1926), G. Nichols (1910), Shimamura (1928, 1929), 

 Sm61ska (1927), Wakayama (1929), W6ycicki (1923a6), Herzfeld (1927). 



