282 SPOROGONIUM OF LIVERWORTS 



analogy to parasitic Angiosperms. As the embryo enlarges, the 

 venter of the archegonium stretches and grows into a protective 

 covering, the calyptra, which can often be recognised around 

 the young sporogonium (Fig. 155, B ; and Fig, 156, I, c). 



In Liverworts the dividing ovum first forms a spherical mass 

 of cells which later elongates and graduallj' becomes differen- 

 tiated into three regions. The end adjacent to the thallus 

 penetrates into the underlying tissue as a cone-shaped sucker, 

 the foot (Fig. 155, B,/.). This consists of parenchymatous cells, 

 and serves to absorb and transfer food to the developing sporo- 

 gonium. The opposite end swells into a, usually, spherical 

 capsule (ca.) with a wall of one or more layers enclosing a mass 

 (the archcsporiiim) of spore-forming cells. Up to a certain stage 

 all the latter are alike, but the archesporium ultimately differ- 

 entiates into two kinds of cells, which at hrst are often arranged 

 in radiating series. One type is rounded, and, since each of 

 these cells divides to form four spores (Fig. 155, C, sp.), they are 

 known as spore-mother cells. The other cells are elongated and 

 at first thin-walled, when they assist in the translocation of 

 food-materuils to the developing spores, but subsequentl}/ thev 

 acquire spiral thickening bands, and form so-called clatcrs 

 (Fig. 155, C, e. ; see also Fig. 147, D). 



The region between capsule and foot, consisting of small 

 cells, often densely crowded with starch-grains, constitutes the 

 seta or stalk (Fig. 155, B, s.), which always remains short until 

 the spores are mature. At this stage, ho«-e\'er, m PelUa and 

 most leafy Liverworts, the stalk elongates rapidly, owing to a 

 great increase in the length of its cells at the expense of the 

 starch-reserves, and bursts through the cah'ptra, which can be 

 recognised as a torn sheath around its base (Fig. 155, A, c.). 

 The capsule is thus raised abo\'e the damp soil into the less 

 humid air, where dispersal of the spores by air-currents is more 

 readily attained. WLere, as in Marchantia. the archegonia are 

 borne on a special receptacle, the stalk of the latter elongates 

 after fertilisation (cf. abo^•e), and scr\-es the same purpose as the 

 seta of other Liverworts, whilst the sporogonium retains the 

 short stalk of the embryonic phase. 



In Pellid and most leafy Li\-erworts the capsule-wall splits 

 into four petal-like valves (Fig. 147, C), although m Marchantia. 



