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A TEXTBOOK OF THEORETICAL BOTANY 



It is very important, however, to realize that the Hfe-cycle of Pinus follows 

 the same basic sequence as that of the previous types we have considered, 

 and that the differences are those of modification rather than the occurrence 

 of any fundamental change. 



The life-cycle of Pinus may be graphically represented by Fig. 696. 



Relationships and Phylogeny 



The relationship of Pinus to other Gymnosperms has been a matter of 

 much discussion. Opinion has been divided as to the most primitive Coni- 

 ferous type, between the Abietineae, of which Pinus is a member, and the 

 Araucariineae, which have simple cone scales and broad leaves. The tracheids 

 in the latter group have several alternating rows of closely contiguous, hexagonal 



A B 



Fig. 697. — Comparison of the type of tracheidal 

 pitting in A, Abietineae. B, Araucariineae. 

 {After Thompson.) 



pits, while Pinus has either single rows or a few rows of circular, separated 

 pits which are opposite, not alternate, to each other (Fig. 697). They also 

 show thickened margins of pectose called " Rims of Sanio " after their 

 discoverer (Fig. 698). The two types are readily distinguishable, even in 

 the fossil state, and both can be traced back to the Palaeozoic. The Araucarian 

 type is certainly nearer to that of the Cordaitales, but the value of this evidence 

 is doubtful, for a good deal of variation occurs. Jeffrey, on the other hand, 

 claims that the transfusion tissue in the Pinus leaf is directly related to that 

 in the leaf of Cordaites, though the needles of the Pine are plainly reduced 

 structures and borne on reduced shoots, while the flat leaves of Araucaria 

 are superficially more akin to those of Cordaitean shoots. 



A double leaf trace is characteristic of the Palaeozoic Gymnosperms. It 

 is also found in Araucaria and to a lesser degree in Pinus, where it may be 

 secondary in origin, as it is not present in the cotyledons as it is in Araucaria. 



