200 EMBRYOGENESIS IN PLANTS 



however, attain to full maturity. Buchholz (1931) regards the embryo 

 of Sciadopitys as a type to which the embryos of the Cupressineae and 

 Taxodineae may be referred. 



Sequoia sempervirens. Fig. 44c-E, shows an unusual, indeed, quite 

 exceptional, embryonic development. The archegonium and ovum are 

 small, the spindle of the dividing zygote nucleus is aligned in the axis 

 of the archegonium, and a transverse wall is laid down. There is, in 

 fact, no free nuclear phase in this genus. The two nuclei again divide, 

 their products being separated by longitudinal walls. Some slight 

 variations have been recorded in the initial segmentation pattern 

 (Buchholz, 1939), Fig. 44c. Each of the four cells can function as an 

 embryo initial and give rise to a filamentous embryo. Some observa- 

 tions seem to indicate the presence of an apical cell, others that the 

 division of the embryonic cell is typically by a longitudinal wall. The 

 subsequent development is characterised by typical cleavage poly- 

 embryony, by the great elongation of the suspensor, the formation of a 

 secondary suspensor, the development of embryonal tubes, and by 

 embryonic growth initially by a single apical cell, this being soon trans- 

 formed into a more massive apex. Rosette cells are usually absent. 

 The single embryo which comes to maturity has usually two cotyledons 

 {see below). The later stages in the embryogeny of Sequoia, in short, 

 are of the general gymnospermous character. 



In Sequoiadeudron giganteum {Sequoia gigantea), the first walls 

 appear when eight free nuclei, which fill the lower half of the arche- 

 gonium, have been formed. Simple and cleavage polyembryony 

 may both be present. A rosette is also conspicuous. The young 

 embryo grows by an apical cell, but this is soon replaced by a group 

 of initial cells. The subsequent developments are generally like those 

 in Sequoia sempervirens. When in due course the single embryo has 

 become considerably enlarged, it is cylindrical in shape and has a 

 massive distal region consisting of the shoot apex and two to six, 

 usually four, cotyledon rudiments {see below). The embryonic develop- 

 ment is very protracted in this species, some two years elapsing between 

 fertilisation and the maturation of the seed. Fertilisation in Sequoia- 

 deudron giganteum takes place in August. The embryo is still in a very 

 immature state at the onset of winter but develops to maturity during 

 the following season (Buchholz, 1937). During the first month the 

 embryo is very small : at the two-celled stage after the division of the 

 zygote, Buchholz estimates that it is 20,000 cubic microns. As the large 

 tree which develops may eventually have a volume of 1640 cubic metres, 

 the organism thus enlarges 82 x 10'"^ times. 



In a statistical study of cotyledon number in Sequoia and Sequoia- 

 dendron, Buchholz (1940) has shown that in the former the range is 



