26 EMBRYOGENESIS IN PLANTS 



the factors which determine polarity, the position of the first wall, the 

 differentiation of a suspensor, and the positional relationship of the 

 enlarging embryo to the parent prothallus. 



THE SEGMENTATION PATTERN 



A distinctive segmentation pattern is characteristic of the early 

 embryogeny in all classes of plants. In some instances, as in the 

 simpler algae, this segmentation consists merely in the transverse 

 partitioning of an elongating filament. In other algae there is also an 

 initial filamentous stage, but divisions in other planes soon follow and 

 a regular histological pattern resuks. In bryophytes, pteridophytes and 

 seed plants the successive segmentations may take place with such a 

 high degree of regularity as to suggest a stepwise (or stage-by-stage) 

 genie control of development. 



In pteridophyte species which have no suspensor, and in the further 

 development of the embryonic cell in those with a suspensor, the cell 

 divisions in the enlarging embryo usually conform to one or two very 

 regular and characteristic histological patterns, Fig. 6. At one time, 

 such cellular patterns were thought to be significant as a guide to 

 taxonomic affinities and phylogenetic relationships. To some extent, 

 because ultimately they are determined in a system with a gene- 

 controlled metabolism, it may well be that they do have this significance. 

 A detached scrutiny of the facts, however, would seem to support 

 D'Arcy Thompson's view that the segmentation pattern in a growing 

 tissue is primarily due to physical factors. A growing embryo, hke any 

 other physical system, will constantly tend towards a state of minimal 

 free energy. Cell division, by keeping constant the ratio of surface to 



Fig. 6. The development of the segmentation pattern in the 

 embryogeny in different groups 

 A-D, Antithamnion plumida, germinating spores (after Kylin). E, Fucus vesiculosiis, 

 young embryo (after Thuret and Oltmanns). F, Cystoclouium purpurascens, the 

 subspherical spore has divided into two and then into four equal quadrants (after 

 Kylin). G, 7V7Ag/o/»'fl/o77o;)/n7/a, early segmentation pattern (after Campbell). H-L, 

 Lejeunia serpliyllifolia (Jungermanniales), spore germination leading to organisation 

 of an apical cell (after Goebel). M, Blechmtm spicaiii (leptosporangiate fern), spore 

 germination, leading to organisation of an apical cell. N, O, P, Diagrams illus- 

 trating the ideal segmentation pattern in a dividing sphere (or circle) and its quad- 

 rants; each cell is equally divided by a wall of minimal area; I-I, II-II, III-III, 

 IV-IV, the successive partition walls. P, Shows the characteristic readjustment at 

 points of wall conjunction (after D'Arcy Thompson). Q, Marsilea vestita, showing 

 a segmentation pattern in the developing embryo that approximates to the ideal 

 pattern illustrated in P (after Campbell). R, Equisetum arvense, young embryo; 

 b-b, basal wall; m-m, first vertical wall; h', root initial (after Sadebeck). S, Epi- 

 pactis pains tris (Orchidaceae), young and fully formed embryos (after Treub). T, 

 Drosera rotundifoUa (after Soueges). U, Chenopodium bonus-henricus (after Soueges). 

 V, Scapaitia nemorosa (Jungermanniales), young sporophyte (after Leitgeb). 



