224 Th e Anatomy of Plants BOOK n 



spaces containing air occur between the elements of the 

 rays, and that the living cells of the latter communicate 

 by pits with these spaces, which in turn are continuous 

 with the intercellular space-system of the cortex with its 

 lenticels, so that the aeration of the internal wood is 

 secured. He also discovered the special albuminous cells of 

 the phloem medullary rays, and showed that both by their 

 structure and behaviour they replace the companion cells 

 of the sieve tubes of the Angiosperms. He pointed out 

 that these cells are to be met with in the leaves, forming 

 an extension of their bundles, just as the transfusion tissue 

 forms a supplementary tissue to the xylem. He found, 

 too, that the sieve tube has no truly perforated sieve 

 plates; the plugs permit the passage of dissolved sub- 

 stances from segment to segment, though they prevent the 

 continuity of the protoplasm. 



Strasburger showed in the same treatise that Pinus at 

 any rate possesses two tissue systems, each extending 

 throughout the whole plant, and being perfectly distinct 

 from the other. They constitute, on the one hand, the 

 region of the stele of the stem, and its communications 

 with the central cylinders of the leaves, and on the other, 

 the tissues of the cortex of the whole, which is pre- 

 eminently an assimilating system. 



In the same work Strasburger showed that in their 

 vegetative structure the Gnetaceae approximate rather to 

 the Dicotyledons than to the other Gymnosperms. 



Many other writers elucidated to a greater or less degree 

 the anatomical problems of the group. Von Mohl in 1871 

 explained the curious transfusion tissue of the leaves of 

 Conifers, though the elements composing it had been des- 

 cribed seven years earlier by Frank. Worsdell investigated 

 the anatomy of the stems of Cycads in 1896 and their 

 sporophylls in 1898, while Scott carried out researches on 

 their peduncles in 1897. 



