CHAP. IT The Differentiation of the Tissues 221 



His work was followed and amplified by the researches 

 of Petersen in 1882, and of J. E. Weiss in 1883. Solereder 

 and Herail independently made important contributions to 

 our knowledge of this distribution two years later. 



The occurrence of phloem-strands in the midst of the 

 tissue of the secondary wood was demonstrated and their 

 formation traced by Morot and by Herail in 1885. Scott 

 and Brebner in 1889, working on Strychnos, showed that 

 these islands, as they called them, are formed like normal 

 phloem, centripetally outside the cambium, and subsequently 

 become enclosed in the wood by means of a new cambial 

 arc completing the normal zone and shutting them in. 



Beside the work on sieve tubes already alluded to, which 

 had a bearing mainly on the continuity of protoplasm 

 through the sieve plate, certain observations by Wilhelm 

 call for comment. In 1880 he showed that some sieve-tubes 

 traverse the medullary rays and afford horizontal com- 

 munication across the tissue. Fischer in 1884 discovered 

 in certain plants of the Cucurbitaceae a transitory system 

 of sieve tubes lying externally to the nbro-vascular bundles. 



In 1883 Weiss demonstrated the formation of tertiary 

 bundles in the xylem parenchyma of fleshy roots. 



The abnormal thickening of the stems of Yucca and 

 other Monocotyledons, by means of a secondary cambium 

 layer, was elucidated by Millardet in 1865. In 1891 Kny 

 made a contribution to our knowledge of abnormal bundles 

 occurring in the group. In 1893 Scott and Brebner investi- 

 gated the structure of the vessels of Dracaena and their 

 method of elongation by means of sliding growth. 



The details of the formation of cork from a merismatic 

 layer or phellogen were first investigated by Sanio in 1860. 

 Later work on the part played by cork in the construction 

 of bark was contributed by Von Hohnel in 1877. To him 

 we owe the recognition of the part played by the pericycle 

 in the formation of the bark of roots. Stahl first studied 



