^64 PHYSIOLOGICAL DEVELOPMENT. 



develop in any systematic way, but branch out irregularly, 

 following everywhere the irregular lobes of the frond 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 w r hich 

 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 

 rcay 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 Phtenogams 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 fibrous structures in antici- 

 pation of their pre-determined functions But in places 

 where new vessels are required in adaptation to a modify- 

 ing 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 transformation of such elongated cells ; and we 

 also see that these bundles of elongated cells have an 

 arrangement quite suggestive of their formation by passing 

 currents. 



Are there grounds for thinking that these further trans- 

 tor mations by which strings of elongated cells pass into 



