690 



A TEXTBOOK OF THEORETICAL BOTANY 



cotyledons. The number may be reduced by fusions taking place, but 

 it seems that polycotyledony is the primitive condition. In the early stages 



the embryo has a single apical cell, which 

 recalls the embryonic development in Pteri- 

 dophyta, but it is later replaced by a group of 

 meristematic initials. 



The central tissue of the female prothallus 

 breaks down as its food materials are taken up 

 by the embryo, leaving a space which is filled 

 by the folds of the continually growing sus- 

 pensor. When the embryo is mature the 

 suspensor dries up and forms a thick cap 

 over the root end of the embryo. The outer 

 layers of the prothallus persist in the mature 

 seed, surrounding the embryo with a nutri- 

 tive tissue called the endosperm. 



The integument of the ovule begins at an 



early stage to form a median layer of stone 



the Abietineae. A, Cleavage cells, and by the time the seed is mature 



polvembrvony with four in- , . . , r r 1^ 



dependent competing embryos, the mner and outer layers ot sott cells 

 B, All four embryonal cells have disappeared, leaving only the stony 



united to form one embryo. , ^ • ^ r 1 • . . 



layer, which lorms the covermg or testa 

 The seed therefore consists of 



only the stony testa, the endosperm and the 



embryo. 



In the third year the female cone reaches maturity (Fig. 692). It is now a 

 dry, brown, woody structure, bearing a pair of mature seeds on the surface 

 of each ovuliferous scale. The seed has a thin wing which assists it in dis- 

 semination, this structure being derived from the surface of the ovuliferous 

 scale. When the seeds are mature the cone scales spring apart explosively 

 with cracking sounds, the separation being due to renewed growth in the 

 cone axis. The seeds are liberated and are blown away by the wind as they 

 fall from the cones. 



Fig. 691. — Diagram illustrating two 

 conditions of polyembryony in 



In the course of germinal com- 

 petition between B and A, B of the seed 

 will alwavs survive. {After 

 Buchholz.) 



Germination 



Pine seeds may germinate at once if they fall where they can absorb 

 water, and in any case their dormant period is not long. The seed coat splits 

 open and the radicle grows downwards into the soil while the plumule grows 

 upwards towards the light (Fig. 693). The cotyledons become green, in fact 

 this may occur before they are exposed to light. They carry up with them the 

 remainder of the seed, and the tips of the cotyledons absorb what is left of 

 the endosperm. The radicle passes downwards into the soil forming a primary 

 tap root, while the developing plumule produces a shoot of unlimited growth 

 upon which delicate needle leaves are produced in a spiral arrangement 

 (Fig. 694). This condition, which may be regarded as primitive for the 



