86 



MORPHOLOGY OF TISSUES. 



diflicult to follow; still more so in the oleander, where also the contents are more 



limpid. 



A Coalescence of Cells arises from the tissue-like union 

 of similar cells, until their contents completely coalesce, 

 the partition-walls becoming partially or wholly ab- 

 sorbed. In this manner are formed those filiform ag- 

 gregates of intercommunicating cells, constituting tubes 

 filled with air or sap, which are known as Fessels, such as 

 Wood-vessels, Bast-vessels or Sieve-tubes, and Latici- 

 ferous Vessels. But roundish groups of cells may also 

 coalesce by the absorption of their walls and the forma- 

 tion of a single large cavity filled with sap ; these are 

 included under the general term Glands (or may be 

 specially distinguished as Compound Glands). Just as the 

 distinction between idioblasts and true tissue-cells is 

 only a progressive one, depending on the augmentation 

 of certain characters, so a coalescence is only an extreme 

 case of the ordinary behaviour of adjacent cells; the 

 contents of these mingle to a certain extent through 

 their partition-walls by diffusion (osmose). Hence we 

 frequently meet with tissues consisting of peculiar cells, 

 which behave physiologically as if they had coalesced, 

 although it is questionable whether the cavities of the 

 cells are actually in communication one with another. 



True Latic'iferous Vessels are composed of coalescent 

 cells containing latex and endowed with the same pro- 

 perties as we have already described as belonging to the 

 laticiferous cells of Euphorbiaceae, Moreae, Apocynaceae, 

 and Asclepiadeae. As far as can be judged from obser- 

 vations which are not yet brought to a conclusion, these 

 vessels originate from rows of cells in the young tissue, 

 and especially in the fibro-vascular bundles, coalescing 

 at an early stage by the complete disappearance of 

 their transverse septa; long tubes (as shown in Fig. 

 72) being thus formed filled with latex, which usually 

 anastomose with one another laterally, and traverse the 

 whole plant in the form of a continuous system of 

 tubes ^. 



The Cichoriacese, Campanulaceae, and Lobeliaceae 

 possess very perfectly developed laticiferous vessels 

 belonging to the fibro-vascular bundles, which they 

 accompany throughout the whole plant in the form 

 of reticulately anastomosing tubes, imbedded, in the 

 case of Gichoriaceae in the outer, in that of the 

 two other orders in the inner phloem-layer. Their 

 form is best recognised by boiling sections of these 

 plants for some minutes in dilute potash solution ; the 

 anastomosing tubes are then clearly recognised in the 

 transparent tissue (Fig. 72), and it is easy to expose them entirely in large pieces. In 



Fig. 7t.— Laticiferous cells from the end of a 

 branch of Euphorbia spUndens, exposed by 

 maceration ; A the whole of one and part of 

 another laticiferous cell (slightly magnified) ; B a 

 piece of one containing starch-grains of peculiar 

 form (strt ngly magnified). 



^ What follows is founded mainly on Hanstein's researches reported in his Preisschrift, Die 

 Milchsaftgefasse und die verwandten Organe der Rinde. Berlin 1864. See also Dippel, Entstehung 

 der Milchsaftgefasse und deren Stellung in Gefassbiindelsystem, 

 fiir wiss. Bot. vol. V. p. 31. 



Rotterdam 1865. — Vogel in Jahrb. 



