228 



PRIMARl' ARRANGEMENT OF TISSUES. 



in the same direction as the root. Their membranes are usually colourless, thus 

 the surface of the air-containing layer appears white and shining. In water, which it 

 quickly absorbs, the layer becomes transparent ; the internal green cortical parenchyma 

 then becomes apparent. Another cause of the green colour is that in old roots 

 (Vanda furva and Anselia africana, according to Leitgeb) small green algal cells 

 sometimes enter the cavity of the tracheides. When old, the air-containing layer 

 is in many species entirely thrown off (Angrsecum subulatum, Cymbidium ensi- 

 folium, Zygopetalum Mackai, according to Leitgeb ; also Vanda furva), or only the 

 innermost layer remains : the result of this also is that the green colour of the 

 cortical parenchyma becomes visible. 



In Eria stellata the colour of the aerial roots is brown, since the membranes of 

 the tracheides assume a brown colour : in Trichotosia ferox the tracheides of the 



FIG. 91.— Median longitudinal section through the apex of a young root of the same Oncidium as Fig. 90, 



with the same lettering (375). 



four-layered sheath are filled with a reddish-brown mass, which gives the roots a 

 reddish-brown colour; the same sometimes applies to .Cymbidium marginatum. 

 Larger or smaller masses of a loosely coherent black-brown substance were found by 

 Leitgeb in many cases, especially in the inmost layer of cells : and in specially large 

 quantity in Renanthera coccinea. According to Leitgeb, those walls which cover the 

 —^ short thin-walled endodermal cells always show a limited brown coloration. 



With the exception of these last-named cases, air alone, or water obtained from 

 without, is always contained in the tracheides ; the protoplasm and nucleus dis- 

 appear entirely during their development, close to the growing-point. 



During their differentiation the walls of the tracheides, like those of other tracheal 

 organs, become lignified to an extent which varies according to the special case. As 

 regards the form of thickening the greatest variety may be seen, not only in those of 

 different species often closely allied to one another, but also in those of differetit layers 



