REPRODUCTIVE CENLS. 143 



prothallus, and, as a rule, are sunk in the tissues of that structure. 

 Each mature oogonium is a flask-shaped organ, composed of two 

 portions — viz., the oogonium proper, or venter, a spherical recep- 

 tacle sunk in the prothallus, and a canal leading from this to 

 the surface. These two parts arise from a single cell of the 

 under surface of the prothallus, this undergoing division into two, 

 the lower cell being again divided into two. The lowest, or 

 rather the deepest of these, becomes the oogonium proper, 

 whilst the two upper ones undergo divisions at right angles 

 to the former plane, lateral cells being formed, and a passage 

 — the canal — arises between them, along the point of union of 

 the planes of division which contains the axial cells. The 

 protoplasmic contents of three small central cells cut off earl}- 

 from the cytoplasm in the venter of the oogonium 

 form the ventral and neck canal-cells, whilst the remaining 

 large mass in the cavity of the oogonium is the oosphere (see 

 Fig. 107, 6), this being the essential female cell. The neck 

 canal-cells are later on converted into a gelatinous plug, which 

 contains a substance (malic acid) capable of attracting the 

 antherozooids (positive chemotaxis). 



c. Fertilisation and the Formation of the Embryo -sporophyte 

 (Asexual Generation).— Fertilisation is accomplished by the 

 passage of one antherozooid into the oosphere by way of the 

 canal of the oogonium. At a certain period the neck canal-cells 

 secrete a substance (malic acid or an enzyme) which has a 

 powerful attraction for the antherozooids, and one of these bodies 

 finds its way down the canal, passing through the mucilaginous 

 plug which now fills that space. 



After penetrating the oosphere, the antherozooid fuses with 

 the nucleus of the oosphere, and this body, being thus fertilised, 

 becomes the oospore. 



The oospore is soon divided by an oblique wall into an 

 epibasal and a hypobasal cell. A second wall at right angles 

 to the first is then formed, and of the four cells now present, the 

 two upper ones go to produce the stem (rhizome) and first leaf 

 of the sporophyte, whilst the lower two give rise to the root, 

 and an absorbing organ, the foot. The foot remains sunk in 

 the prothallus whilst the first leaf grows upwards, usually 

 through the notch of the prothallus, the rhizome and root 

 growing horizontally and downwards respectively. In con- 



