262 PART III. THE CLASSIFICATION OF PLANTS. 



an alternation of generations, the asexual form being the sporophyte. It is 

 also probable that in the case of those forms of which only individuals bearing 

 either asexual (e.g. Desmarestia, Laminaria) or sexual (e.g. Scytosiphon, 

 Phyllitis, Colpomenia) organs are known, the life-history may present an alter- 

 nation of generations. 



The Phaeosporeffi are almost exclusively marine, the only fresh- water forms 

 being the genus Pleurocladia (Ectocarpaceae) and two species of the genus 

 Lithoderma. The size of the plants included in this series varies widely, from 

 microscopic Ectocarpaceae to gigantic tree-like Laminariacese, such as Macro- 

 cystis, Nereocystis, etc., which may attain a length of several hundred feet. In 

 some of theLaminarias, which have cylindrical stalk-like region in their thalloid 

 shoot (see Fig. 159), secondary growth in thickness takes place by means of a 

 merismatic layer. In these large forms, too, the conducting tissue is sometimes 

 so far developed as to form sieve-tubes ; though no woody tissue is developed, 

 nor is it required in view of the fact that these plants live submerged. 



Series PH^OGAM^;. The orders comprised in this group are characterised by 

 the oogamous sexual process, by the absence of gonidia, and by the non- 

 motile spores ; the orders may be conveniently described separately, as they are 

 well defined. 



Order 8. Tilopteridaceae. Body filamentous, differentiated into shoot and 

 root ; the shoot is, in its younger parts, monosiphonous ; in its older parts 

 polysiphonous, but without cortex ; growth in length by means of intercalary 

 growing-points ; lateral members, some with unlimited growth (branches), some 

 with limited growth; the root consists of a single row of cells, and its growth is 

 intercalary, without any special growing-point ; marine. 



The asexual and sexual reproductive organs are borne on distinct individuals. 

 The asexual organs are terminal, or less commonly intercalary, unicellular 

 sporangia ; the sexual organs are unicellular oogonia, and multicellular an- 

 theridia (resembling the gametangia of the Phaeosporeae), and are intercalary 

 The sporangium gives rise to a single non-motile spore, which, at the time 

 it is set free, has a delicate cell-wall, and contains four nuclei. The oogonium 

 (which somewLa!; resembles the sporangium in form) gives rise to a single 

 oosphere, which, when it leaves the oogonium, has no cell-wall. Each cell 

 of the antheridium gives rise to a single spermatozoid, oval in form, with 

 two cilia. The sexual process and the germination of the oospore have not 

 been observed. The spore, on germination, undergoes repeated division, 

 forming a solid multicellular body, from which a long root grows out ; from 

 the multicellular embryo spring the adult shoots. 



The life-history of the Tilopteridaceas probably presents an alternation of 

 generations, the form bearing the asexual organs being the sporophyte : for 

 instance, the forms known as Haplospora and Scaphospora are probably the 

 asexual and sexual generations, respectively, of the same plant, a Tilopteris. 



Order 9. Dictyotaceae. Body flattened, ribbon or fan-shaped, sometimes 

 dichotomously branched, consisting of several layers of parenchymatous cells, 

 with a well-marked midrib in Dictyopteris, differentiated into root and shoot ; 

 growth of the shoot takes place in Dictyota by means of a single apical cell 

 (see Fig. 140), in the other frrns by a marginal series of merismatic cells. Both 

 asexual and sexual organs are known. The asexual organs are unicellular 



