76 DICTYOTALES. 



portions which are found floating are always barren, only those 

 attached are fertile. 



USES. The Fucaceae, like the Laminariaceae, are used as manure (the best 

 kinds being Fucus vesiculosus and Ascophyllum nodosum), for burning to pro- 

 duce kelp, and as food for domestic animals (Ascophyllum nodosum is especially 

 used for this purpose). 



Class 9. Dictyotales. 



The plants in this class are multicellular, and brown, with 

 apical growth, new cells being derived either from a flat apical 

 cell, or from a border of apical cells. The thallus is flat, leaf- or 

 strap-shaped, attached by haptera, which are either found only 

 at the base, or on the whole of the lower expansion of the thallus. 

 The cells are differentiated into the following systems of tissues : 

 an external, small-celled layer of assimilating cells, generally one 

 cell in thickness, and an internal, large-celled layer of one or only 

 a few cells in thickness, forming the mechanical and conducting 

 tissues. All the reproductive cells are motionless. Asexual re- 

 production by naked, motionless spores (tetraspores) which are 

 formed 1-4 in each tetrasporangium, the latter being outgrowths 

 from the surface cells of special, sexless individuals. Zoospores 

 are wanting. The sexual organs are of two kinds, oogonia and 

 antheridia, which are formed from the surface cells, either on the 

 same or different individuals. The oogonia are spherical or oval, 

 and are generally placed close together; each contains one 

 oosphere, which on maturity is ejected into the surrounding 

 water, and is then naked and motionless. The antheridia are 

 formed of longitudinal cells, united in groups, whose contents 

 by repeated divisions transverse and longitudinal are divided 

 into a large number of small, colourless, motionless spermatia 

 round or elongated which are set free by the dissolution of the 

 wall of the antheridium. The process of fertilisation has not 

 yet been observed. 



The Dictyotales, in having^ tetraspores and spermatia, deviate 

 -considerably from the Phseophyceae, but may be classed near to 

 the Tilopteridge, in which there are asexual spores with 4 cell- 

 nuclei, which may be considered as an indication of the formation 

 -of tetraspores. 



Order 1. Dictyotacese. Dictyota dichotoma which has a thin, regularly 

 .dichotomously divided thallus, occurs on the coasts of the British Isles 

 Padina is found on the south coast. 



