8 Miyake, The development of the gametophytes etc. 



Carothers (1907) reports that in the early stag'es of the growing 

 female prothallium of Ginkgo, the division of the free nuclei is 

 simultaneous, but later free nuclear divisions proceed irregularly. 

 The question whether this is normally so in Ginkgo or represents 

 rather an abnormal case, remains to be investigated. Her material 

 was subjected to "unnatural condition — growth after Separation 

 from the tree — although an effort was made to render conditions 

 as natural as possible", and the author herself admits that the 

 latter alternative is not impossible. 



It is usual in the Coniferae, that in each ovule, a Single 

 megaspore develops into the female prothallium, and I have never 

 found a case in Cmminghamia where the ovule contained more 

 than one prothallium. Arnoldi (1900), however, found that more than 

 one prothallium are sometimes developed in the ovule of Cimning- 

 Jmniia, and figures as many as five young prothallia of various sizes in a 

 Single ovule (Arnoldi, 1900, texfig. 5). In Oryptomeria Lawson 

 (1904 b) found that only one out of twelve or sixteen potential 

 megaspores germinates and there is consequently but one pro- 

 thallium formed. Coker (1903) found that in Taxodium only a 

 Single prothallium is usually formed, but one case was found in 

 which the nucellus contained two young prothallia. Hofmeister 

 (1851) mentions the occasional presence of two prothallia in Pinus 

 sylvestris and Taxus haccata. It was since confirmed by Farmer 

 (1892) for Pinus and by Strasburger (1904) and Coker (1904) 

 for Taxus. Coker (1902) has also found two prothallia in the 

 young ovule of Podocarpu^. In Sequoia (Arnoldi, 1899a, 1899b; 

 Lawson, 1904a) there are usually more than one prothallium or- 

 ganized, and as many as eight has been counted in a Single ovule; 

 one of them enlarges more rapidly than others and form the pri- 

 mary prothallium, while one or two secondary prothallia, though 

 failing to produce true prothallial tissue, nevertheless reach an 

 advanced stage of development. In this respect Sequoia differs 

 from other Conifers where the single prothallium seems to be the rule. 



Early in the development of the young prothallium, it is 

 surrounded by two to four layers of larger cells or tapetum. Each 

 cell of the tapetum has a larger nucleus and denser cytoplasm 

 when compared with the surrounding cells of the nucellus (figs. 46, 

 108). The presence of the tapetum-tissue in Ounninghamia has 

 already been observed by Arnoldi and is flgured in his paper 

 (Arnoldi, 1900, textfigs. 4 and 5, PI. XVII, fig. 2). A simüar ta- 

 petum was found in Taxodium by Coker (1903), while it is re- 

 ported by Lawson (1904b) to be abseut in Cryptomcria. 



The origin of the tapetal layer has been differently interpretted 

 by various investigators. Lang (1901) designates a similar tissue 

 in Stangeria as sporogenous cells. Thomson (1905) considers that 

 in aU Gymnosperms ^^'hose megaspore-membrane belongs to a normal 

 type have the tapetum originated from the sporogenous tissue. 

 Noren (1907) agrees with the above mentioned authors in re- 

 garding the tapetum-cells in Juniperu^ as sporogenous in origin. 

 Coker (1903) on the other hand, is inclined to take the tapetal 



