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BOTANY 



PART 11 



four sterile jieriijheral cells, which completely enclose two central sperniatogenous 

 cells. Froin each of the latter arise, in turn, two spermatozoid mother-cells, four 

 in all, each of which, when liberated by the rupture of the spore wall, gives rise 

 to a single, spirally coiled, multiciliate spermatozoid. The female prothallium 

 (Fig. 420) just as in Sclaginella, also remains enclosed within the macrospore, and 

 is incapable of independent growth. It shows similarly an approach to the 



Fio. 420. — Isoetes echinospora. A, Female prothallium ; 

 fir, archegonium ; o, egg-cell. B, C, development 

 of the archegouium from a superficial cell ; h, neck- 

 cells ; hk, neck canal-cell ; b, ventral canal-cell ; 

 0, egg-cell. (After Campbell, x 250.) 



Fio. 421. — /yoe/e.s echinnspitra. Embryo before 

 breaking out from the prothallus in longitudinal 

 section, cot, cotyledon ; I, ligule ; r, sheath at 

 the base of the cotyledon in the axil of which 

 the apex of the stem arises ; iv, root ; /, foot. 

 (X 20n. After Campbell.) 



Conifers, in that the nucleus first divides into numerous, parietal daughter-nuclei 

 before the gradual formation of the cell walls, which takes place from the apex of 

 the spore to the base. As a result of this process the whole spore becomes filled 

 with a prothallium, at the apex of which the archegonia are developed. The 

 embryo has no suspensor and thus differs from other Lycopodinae. 



The Fossil Cryptogams (^^-) 



The remains of cryptogamic plants of former geological periods afl'ord no 

 evidence as to the phylogeuetic relations of the classes of Thallophyta and 

 Bryophyta. Intermediate forms between Algae and Archegoniatae, are, as yet, 

 unknown. On the other hand Phytopalaeontology has made us acquainted with 

 interesting, long-extinct types of Pteridoi)hytes, which serve to complete the 

 classification of the existing Ferns, Horse-tails, and Club-mosses, and in part 

 afford a transition from the Ferns to the Gymnosperms. 



I. The great majority of Thallophytes are, on account of their delicate structure, 

 not adapted to be preserved in the fossil condition. The absence of remains of 

 many classes of Thallophytes is therefore no ground for concluding that they did 

 not exist in earlier ])eriods. Even in the Silurian rocks remains of Algae are 

 found, though they do not permit of their relationship being determined. The 

 remains of calcareous Algae belonging to the Siphonales are, owing to their good 

 preservation, among the best known ; of these, forms are known from the Tertiary 

 strata back to the Silurian, while the Corallineae, which are calcareous Red Sea- 

 weeds appear from the upper Jurassic onwards. Among unicellular Algae the 

 Diatomaceae, which have a silicified cell wall, are well preserved. The fossil forms 



