lyo TRACHEM. 



explained in plants without laticiferous tubes, though plausible conjectures' may be 

 made on the subject. The same holds as a matter of fact also for plants with latici- 

 ferous tubes, but there are controversies on this point, which we shall return to in 

 Chap. VI. 



The frequent filling up of the cavity in the Trachea;, e. g. in old layers of wood 

 of Coniferae and many Dicotyledons, with resin or resin-like masses is undoubtedly a 

 phenomenon of incipient degradation and disorganisation. It will be further treated 

 of in Chap. XIV. 



In old or damaged, large, tubular Trachea; the internal cavity is not unfrequently 

 partially or completely filled with parenchymatous cells (Fullzellen), which in the 

 wood of the chestnut drew the attention of Malpighi^: they have since been 

 frequently described, and have been termed by the anonymous writer in the 

 Botanische Zeitung ^ lliyloses (Thyllen). 



They may arise where a Trachea borders on parenchymatous cells, and in fact 

 from those cells themselves, which grow into it. A small part of the membrane of a 

 parenchymatous cell adjoining an unthickened point on the wall of a Trachea (as a 

 rule a pit) grows to an excrescence protruding into the cavity of the latter : it 

 contains protoplasm, usually with a well-marked nucleus, and expands from a bluut 

 and short cylindrical form to a round, often voluminous bladder, and finally cuts 

 itself off as a special cell from the rest of the cavity of the cell which produced it by 

 means of a division-wall, formed at its point of entrance into the Trachea. Thus there 

 always arise at first solitary bladder-like cells protruding from the wall into the cavity 

 of the tube. The process may be arrested at this point : but often the phenomenon 

 is extended quickly over numerous points of a portion of a tube, so that the latter 

 gradually becomes entirely coated internally with the cells, and these, as they extend, 

 gradually fill it up completely. This often happens to such an extent that the tube 

 is entirely filled by thy loses flattened into polyhedral forms by reciprocal pressure. 

 Further, a multiplication of them by division has been observed in many cases ^ 



The parenchymatous cells bordering on a tube take part unequally without 

 recognisable rule in the formation of thyloses : some throw out thyloses at one 

 point, others at several, others again not at all. The formation of fresh thyloses may 

 continue for a long tim.e in a portion of a vessel : in vessels several years old (e. g. in 

 an eight years' old layer of wood of Vitis, Reess, /. c.) the first beginnings of new 

 thyloses often occur in close proximity to others apparently several years old. 



The cellulose wall of the thyloses, which is at first delicate, is later thickened in 

 woody plants, and often has corresponding pits on the surfaces of contact with other 

 thyloses. In the same plants starch may be stored up in their contents, as is the 

 case in normal parenchymatous cells. 



The formation of thyloses has been observed in ^Monocotyledons (Arundo 



' Anat. Plant, p. 9, Tab. VI. fig. 23. 



* Vol. for 1845, p. 225. 0;5AA.is = sac, bag, reservoir. In this treatise the older literature on the 

 subject is referred to. For more recent information see Reess, Zur Kritik der Bohm'schen Ansicht 

 iiber die Thyllen, Bolan. Zeitg. 1868, p. i ; Unger, Ueber d. AusfuUung altenider u. verletzter Spiralge- 

 fasse durch Zellgewebe, Sitzungsber, d. Wiener Acad. Bd. 56 (1867). 



^ Trecul, Sur Torigine des bourgeons adventifs; Ann. Sci. Nat. 3 ser. torn. VIII ^^Maclura). — A. 

 Gris, Ann. Sci. Nat. 5 ser. torn. XIV. p. 38 (Cissus). 



