THYLOSES. 



171 



Donax, Canna, Hedychium, Strelitzia, Musa, Palms), and in the wood of very many 

 Dicotyledons, both in one-year-old stems (Canna, Cucurbita, Bryonia, Cucumis, 

 Solanum tuberosum), Euphorbia helioscopia, &c., and especially in long-lived stems 

 of Dicotyledonous woody plants, where they are very widely distributed, and easily 

 observed phenomena, e. g. in Vitis, Quercus, Sambucus, Platanus, Robinia, &c. But 

 in the roots of Dicotyledonous trees, which have been examined for them (Quercus, 

 Fraxinus, Fagus, Betula, &c.), they do not occur, or extremely rarely ' : in .the roots 

 of herbaceous plants, however, they occur in large quantity : Pharbitis hispida, young 

 strong roots of Cucurbita, Urtica, Rubia, &c. 



The tubes in which thyloses appear are in most cases typical, wide-pitted 

 vessels : but in Canna (and also in Musa and its allies) they are also the above- 

 mentioned (p. 165) wide, fibrously-thickened non-perforated, tracheides. 



In the pitted vessels of many Dicotyledonous woody plants the formation of 

 thyloses is a regular phenomenon, which appears in the normal uninjured plant, 

 though not extending to all pitted vessels. In Robinia pseudacacia it is stated that 

 the pitted vessels of the wood (and, according to Gris^ all the vessels) begin to be 

 filled with thyloses in the autumn of the year in the spring of which they were 

 formed, and that these are at times filled with starch. Other woods behave in the 

 same way as regards the time of first formation of thyloses, but no definite rule has 

 been recognised for their occurrence or absence ; e. g. in Vitis, Quercus robur, 

 Platanus, according to Reess. Injuries by which the vessels are opened are, as far 

 as investigated, without influence on the formation of thyloses in woody plants. In 

 the large tracheides of the stem of Canna, however, they occur, according to Unger, 

 if these have been injured, e. g. cut into, and then exposed .to air or water. These 

 facts may afford starting-points for the inquiry into the still unknown causes of the 

 formation of thyloses, which cannot be further noticed here. 



' Von Mohl, Botan. Zeitg. 1859, p. 294, aod ^earlier. 



