Lecture XVII. 133 



filamentous plant. Divisions are soon developed which transform 

 it into a plate of cells attached to the soil with rhizoids. Growth 

 and differentiation proceed till a diminutive plant of Marchantia 

 is formed. 



Comparison of the life-history of Marchantia with that of a 

 simpler organism like Fucus brings out a striking difference. The 

 oosperm of Fucus grows and differentiates directly into an indi- 

 vidual which resembles the parents which produced the ovum 

 and sperm. This is not the case with Marchantia. The fertilised 

 ovum grows into the small structure composed of sporangium, 

 stalk and foot, which develops in the archegonium on the arche- 

 goniophore. The growth of this structure takes place at the 

 expense of the materials in the archegoniophore, which are ab- 

 sorbed and transmitted to it by the foot. This spore-producing 

 apparatus is entirely dependent and parasitic on the archegonio- 

 phore. Plants, like the parents which produced the ovum and 

 sperm, are only formed when the spores germinate and develop. 

 Compared with Fucus the spore-producing apparatus is a new 

 structure introduced into the life-history, and without parallel in 

 the life-history of the simpler plant. The plant which produces 

 the gametes or sexual cells is appropriately called the gametophyte, 

 while the individual which develops from the oosperm and pro- 

 duces spores is called the sporophyte. The complete life-cycle 

 of Marchantia comprises an alternation of these two phases. The 

 first phase is the gametophyte. It consists of the creeping 

 bifurcating thallus, which, absorbing water and mineral salts from 

 the soil by its rhizoids, constructs organic substances by photo- 

 synthesis in its green tissues. For reproduction it forms arche- 

 goniophores and antheridiophores, producing archegonia and 

 antheridia with their ova and sperms. The second phase is the 

 sporophyte, much simpler in structure, composed of the sporangium, 

 its stalk and foot, and wholly parasitic on the gametophyte. 



Evidently such a life-history is more comparable to that of 

 Polysiphonia. In the latter we have a somewhat similar alter- 

 nation of carposporic phase the gametophyte with the tetra- 

 sporic phase the sporophyte. 



Another point of difference between the true thallophytes, which, 

 as we have seen, are primitively aquatic plants, and the repre- 

 sentatives of terrestrial plants like Marchantia, is to be found in 

 the structure of the sexual organs. The gametangia or receptacles 

 of the sexual cells in the former are bounded by the cell-wall of 

 a single cell within which the gametes develop. In the higher 

 types the gametangia are multicellular and are built up, like the 



