MECHANICAL ACTION OF BRANCHES, ETC. 581 



older they become. While the young branches of the Lime tend upwards, the older 

 branches of the same tree fall in curves towards the earth. The same is seen in the 

 older branches of the Pines. The youngest lignified branches at the apex of the 

 Fir Ftcea exceha ascend obliquely into the air, further down the branches are 

 horizontal, and the lowermost even curve downwards.' This phenomenon is still 

 more evident in vigorous specimens of Pinus austriaca: the tender annual shoot 

 at the end of each horizontal branch stands erect, then becomes curved a little 

 obliquely ouhvards, and in the course of the year becomes more and more oblique, 

 and then when the new shoot is developed next year — again vertical — the last 

 year's piece of branch sinks till nearly horizontal, to become completely so in 

 subsequent years. Thus it is that a horizontally extended branch of this iree is 

 composed of distinct annual shoots, each of which at first stood perpendicularly 

 erect. 



In these processes occurring in woody branches and stems we have been chiefly 

 concerned with a greater or less obstruction of growth, by means of the mutual 

 pressure of layers of tissue. I may now add a few examples showing how vitally 

 active cells begin to grow anew on being relieved from pressure. 



One of the finest examples in this connection is presented by the formation of 

 so-called Tyloses^ in the vessels of the wood of Dicotyledons. If the vessels, 

 especially the wide ones, of Robinia, the Oak, Vine, and many other woods are 

 examined microscopically, when they have attained a certain age, they are found 

 to be entirely filled with a parenchymatous tissue, which was observed and figured 

 even by the first vegetable anatomist, Malpighi, though of course he had no idea 

 of its origin. Not before recent researches, especially the careful labours of Reess 

 (1868), were accomplished, was the remarkable origin of the tyloses rendered clear. 

 They arise in fact by the very thin closing membranes of the bordered pits, at the 

 spots where the vessels abut on soft parenchyma cells, becoming forced into the 

 cavity of the vessel under the turgescence of the latter, and then beginning to 

 grow vigorously. A club-shaped vesicle is thus formed, which, as it grows, 

 undergoes cell-divisions, and when such structures protrude from numerous pits, 

 they fill up the cavity of the vessel and compress one another, and thus produce a 

 parenchyma-like tissue. 



The whole process would be quite impossible if the tube of the vessel itself was 

 filled with sap and turgescent ; but, as it is, the vessel loses its sap, and the air con- 

 tained in it is even rarefied, and thus the turgescence of the neighbouring parench3rma 

 cells must drive forwards the fine closing membranes of the pits into the cavity of the 

 vessel. I observed something of the same kind, but on a larger scale, in 1854, in the 

 internodes of Bean seedlings which had become hollow : the pit|i cells which 

 surrounded the cavity had grown into it in the form of club-shaped or spheroidal" 

 papillae, and had even undergone division several times. The same cells, however, had 

 the pith not been ruptured by the stronger tension of the external layers of tissue,' 

 would have remained as polyhedral parenchyma cells compressing one another on 

 all sides in the usual manner. 



' Max Reess, 'Z«r Kritik der BShnCschen Ansicht uber die EntwicMungsgeschichte und 

 Funktion der Tiillen,'' Bot. Zeit. 1868, p. i. 



