35« 



GYMNOSPERMS 



of the three layers. It becomes partly absorbed and partly crushed 

 by the growing gametophyte, until, in the ripe seed, it is only a 

 thin, papery membrane, somewhat thicker at the top, where it 

 consists of the disorganizing nucellus. 



Many conifer seeds will germinate 

 immediately, without any resting pe- 

 riod; but some of them remain dor- 

 mant for a long time. As already 

 noted, the cone scales of Pinus mu- 

 cronata,Sin endemic California species, 

 may open only when there is a fire, or 

 an unusually w-arm summer. 



At germination the testa is carried 

 up at the tips of the cotyledons, which, 

 at first, are curved because their tips 

 are held together inside the seed coat 



(fig- 342). 



All forms which have spur shoots, 

 like Pinus, or appressed leaves, like 

 Thuja, or phylloclads, like Pliyllo- 

 cladus, have simple needle leaves in 

 the young seedlings. These simple 

 needle leaves also appear in adult 

 plants when a bud develops on ac- 

 count of a wound. 



We believe in the recapitulation 

 theory, but we also believe that it has 

 been called upon to explain things not 

 at all related to it. In support of the 

 theory we should lay the greatest 

 stress upon the life-histories of pteridophytes in their relation to the 

 evolution of the seed. Some features of vegetative anatomy seem best 

 explained in accordance with the theory. We are inclined to believe 

 that the juvenile leaves of conifers are really a recapitulation of an 

 early ancestral condition, and that spurs, appressed leaves, and phyl- 

 loclads came later. 



Fig. 342. — Pinus edidis: A , seed- 

 ling with tips of cotyledons still in- 

 side the seed: B, later stage with 

 cotyledons expanded and young 

 leaves appearing. No spurs at this 

 stage. The portion below the coty- 

 ledons and down to the break is 

 hypocotyl. About two-thirds of the 

 hypocotyl is shown, one-half nat- 

 ural size. 



