254 



MORPHOLOGY OF GYMNOSPERMS 



of cells that disappear with the maturing of the testa. The middle or 

 stony layer in this case is the conspicuous one, the seed being said to 

 " ripen dry " ; while the inner fleshy layer is most largely and distinctly 

 developed, as usual, in the free portion of the integument (fig. 275). 



The vascular supply of the seed has 

 been modified by this change in the 

 structure of the testa. Both inner 

 and outer sets of vascular strands 

 have disappeared, the vascular con- 

 nections ending in a more or less 

 extensive tracheal plate at the base 

 of the ovule. 



The development of the sporogenous 

 tissue has seldom been traced. In 

 Larix Strasburger (25) has demon- 

 strated a hypodermal archesporial cell, 

 which divides periclinally into an outer 

 primary wall cell ("tapetal cell") and 

 an inner primary sporogenous cell (figs. 

 276-279). The wall cell begins an 

 extensive series of divisions, accom- 

 panied by divisions of the overlying 

 epidermal cells, so that soon there is a 

 large mass of sterile nucellar tissue 

 capping the sporogenous cell. Pre- 

 sumably this description applies in a 

 general way to the formation of the 

 nucellar cap in all the forms, but in 

 most cases the hypodermal cells are not distinguishable from the 

 others, and the succession of divisions cannot be followed with any 

 certainty. In Sciadopitys Lawson (175) describes the differentiation 

 of a "pollen cushion" at the tip of the nucellus, consisting of a loose 

 tissue of large thin-walled cells that receives the pollen grains. In gen- 

 eral, the first clear evidence of sporogenous tissue is the appearance of 

 one or more deep-lying mother cells, recognized first by their increasing 

 size. It is probable that if there is a primary sporogenous cell formed 

 as described above, it does not divide to produce sporogenous tissue. 



Fig. 275. — Pinus Laricio: 

 longitudinal section of ovule; o, 

 outer fleshy layer of integument; 

 i, inner fleshy layer; s, stony layer; 

 p, pollen tubes; n, free portion of 

 nucellus; g, gametophyte; a, 

 archegonia. 



