78 EMBRYOGENESIS IN PLANTS 



In Andreaea the spore contents divide in a regular and characteristic 

 manner and form a cellular tissue whilst still enclosed within the intact 

 outer wall. This wall is then ruptured and the young multicellular 

 gametophyte emerges. In the inception of the gametophyte we have 

 thus what is virtually an enclosed embryogeny. A filamentous develop- 

 ment ensues and in due course buds give rise to erect stems as in other 

 mosses. 



As the zygote enlarges, the first division is by an approximately 

 median transverse wall. The developments of the two segments are 

 somewhat different from those in Sphagnum. The first division of the 

 epibasal cell is by a curved oblique wall, as shown in Fig. 18g, the 

 distal portion being considerably larger than the other. If the two cells 

 are in equilibrium, we must suppose that the distribution of metabolites 

 in the epibasal cell before division was far from homogeneous. With 

 the formation of a second curved oblique wall, an apical cell is con- 

 stituted. Similar divisions also take place in the hypobasal cell, with 

 the result that the embryo becomes an elongated spindle-like structure, 

 with an apical cell at either end. In the epibasal region, periclinal walls 

 are next laid down and define the amphithecium and endothecium. On 

 further growth and development, the capsule and seta are differen- 

 tiated, while the hypobasal region becomes specialised as a haustorial 

 foot, Fig. 18k. 



Here we may note that whereas in Sphagnum the archesporium is 

 derived from the inner layer of the amphithecium, the endothecium 

 giving rise to the columella only, in Andreaea, as in the Bryales, the 

 archesporium is formed from the outer layer of the endothecium. 

 While this is not a matter which strictly pertains to the embryology, it 

 provides further evidence that in the precisely regulated development 

 of the bryophyte sporophyte, certain genetical factors apparently 

 become active at particular stages in the ontogeny. 



In a moss such as Funaria, the spore germinates to form simple 

 filaments which emerge from opposite sides (see also p. 64). These 

 filaments divide by transverse walls and branch freely: those which 

 are exposed on the soil surface become photosynthetic and have 

 transverse septa, whereas those which penetrate the soil are colourless, 

 or have brown walls, and typically oblique septa. 



The moss archegonium is typically a distal and free structure, 

 seated on a short or long pedicel through which the embryo is supplied 

 with nutrients from the gametophyte. Acropetal gradients are thus 

 involved in the embryonic development. In the Bryales, the mature 

 ovum is spherical or ovoid with a colourless region on the side towards 

 the neck of the archegonium. The zygote divides by a transverse wall 

 as in Sphagnum and Andreaea. The next developments are closely 



