THE INNER TISSUES OF PLANTS. 281 



develop in any systematic way, but branch out irregularly, 

 following everywhere the irregular lobes of the fronds as 

 these spread ; and on examining under a magnifier the places 

 at which the veins are lost in the cellular tissue, it will be 

 seen that the cells are there slightly longer than those 

 around: suggesting that the lengthening of them which 

 produces an extension of the veins, takes place as fast as 

 the growth of the tissue beyond causes a current to pass 

 through them. In the second place, a disappearance of the 

 granular contents of these cells accompanies their union 

 into a vein — a result which the transmission of a current 

 may not improbably bring about. But be the special causes 

 of this differentiation what they may, the evidence favours 

 very much the conclusion that the general cause is the 

 setting up of a current towards a place where the sap is 

 being consumed. In the histological development 



of the higher plants we find confirmation. The more 



finished distributing canals in Phaenogams are formed of cells 

 previously lengthened. At parts of which the typical struc- 

 ture is fixed, and the development direct, this fact is not easy 

 to trace; the cells rapidly take their elongated structures in 

 anticipation of their pre-determined functions. But in places 

 where new vessels are required in adaptation to a modifying 

 growth, we may clearly trace this succession. The swelling 

 root of a turnip, continually having its vascular system 

 further developed, and the component vessels lengthened as 

 well as multiplied, gives us an opportunity of watching the 

 process. In it we see that the reticulated cells which unite 

 to form ducts, arise in the midst of bundles of cells that have 

 previously become elongated, and that they arise by trans- 

 formation of such elongated cells ; and we also see that these 

 bundles of elongated cells have an arrangement suggestive 

 of their formation by passing currents. 



'Are there grounds for thinking that these further trans- 

 formations by which strings of elongated cells pass into 

 vessels lined with spiral, annular, reticulated, or other 



