THE GYMNOSPERMAE : CONIFERALES AND TAXALES 689 



formation of four tiers. The lowest tier form the embryonal cells, the 

 next above are the suspensors, and the third tier is called the rosette tier. 

 As stated, the uppermost or fourth tier plays no further part. This sym- 

 metrical grouping of the pro-embryonal cells is characteristic of Coniferales 

 as an order, though there are not always four tiers. The suspensors elongate 

 very greatly and the embryonal cells are thrust down into the tissues of the 

 female prothallus, which functions as a nourishing tissue (Fig. 690). It is a 



Remains of 

 archegonium 



Suspensors 



m,\l III Embryos 



■ Prothallus 



Fig. 690. — Piiuis sylvestris. 

 Longitudinal section of 

 the female prothallus 

 showing the penetration 

 of the embryo into the 

 prothallial tissue. 



curious feature of embryogeny in Piniis that the four embryonal cells normally 

 separate from each other and develop independently into four embryos, 

 each of which develops secondary suspensor cells. The whole structure of 

 each embr^'O, therefore, comes from one cell only. Such a condition, which 

 IS widely characteristic of the Coniferales, is termed polyembryony, or more 

 strictly cleavage polyembryony, to distinguish it from the similar condition 

 which may arise from the fertilization of more than one archegonium or by 

 the development of embryos from the rosette cells. In the course of time, 

 however, owing to a competition for food, only one of these embryos survives 

 to come to maturity (Fig. 691). 



The developed embryo is straight, consisting of a short axis with the 

 radicle towards the micropvle and the stem apex or plumule downwards. 

 The plumule is surrounded by a group of about ten primary leaves or 



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