336 



CONDUCTING SYSTEM 



correspond on the two sides, whereas the walls dividing sieve-tubes from 

 leptome-parenchyma cells are as a rule altogether devoid of pits. 



Among Monocotyledons, and also in the primary leptome-strands of 

 certain Dicotyledons, the companion-cells are arranged in continuous 



series. In most Dicotyledons, however, this is 

 not the case, at anyrate as regards the secondary 

 phloem. In Gymnosperms and Pteridophytes 

 these structures are absent. Their place is taken 

 by rows of special parenchymatous elements with 

 very abundant protoplasmic contents ; in the 

 secondary phloem of Gymnosperms these " albu- 

 minous cells " only form continuous series over 

 short distances, in which respect they resemble 

 the companion-cells of Dicotyledons. 



The physiological significance of the com- 

 panion-cells (and of the analogous albuminous 

 cells) is quite unknown. It has already been 

 stated that they do not always form continuous 

 series for long distances ; assuming that they 

 take part in the transportation of protein com- 

 pounds at all, they may, as Strasburger supposes, 

 be concerned with purely local requirements. 

 In view of their close anatomical relation to the 

 sieve-tubes, however, it seems possible that they 

 may co-operate with the latter in some other 

 way, although the precise nature of the assist- 

 ance which they render cannot at present be 

 indicated. They might, for example, conceivably 

 play a leading part in connection with the 

 movement of the liquid contents of the sieve- 

 tubes. A. Fischer believes them to be organs 

 engaged in the synthesis of proteins ; but this 

 view is no more deserving of acceptance than 

 the hypothesis of Sachs, which attributes a 



Fig. 136. 



Small portion of a L.S. 

 through the leptome of the 

 stem of Ecballium Elaterium. 

 c, leptome-parenchyma cell 

 (cambiform cell) ; s, sieve-tube; 

 co, callus-pads deposited on 

 the sieve-plates ; q, companion 

 cell. 



similar function to the sieve-tubes themselves. 



4. Latex-tubes or laticiferovs elements. 



In the great majority of higher plants, the sieve-tubes, the leptome- 

 parenchyma (cambiform tissue) and the conducting parenchyma suffice 

 for the translocation of the whole supply of plastic materials. But 

 certain families of Angiosperms are characterised by the possession of 

 an additional conducting tissue in the shape of laticiferous elements or 

 latex-tubes, structures which owe their name to the milky appearance 



