284 PHYSIOLOGICAL DEVELOPMENT. 



tinuous spiral threads, or networks, or structures between 

 spirals and networks, or networks with openings so far 

 diminished that the cells containing them are distinguished 

 as fenestrated. Their differences omitted, however, these 

 structures have the common character that, while support- 

 ing the coats of the vessels, they also give special facilities 

 for the passage of liquids, both through the sides of the 

 transformed cells and through their united ends, where these 

 are not destroyed. 



To attempt any physical interpretation of this change is 

 scarcely safe: the conditions are so complex. There are 

 reasons for suspecting, however, that it arises from a vacuo- 

 lation of the substance deposited on the cell-wall. If rapidly 

 deposited, as it is likely to be along lines where sap is freely 

 supplied, this may, in passing from the state of a soluble 

 colloid to that of an insoluble colloid, so contract as to leave 

 uncovered spaces on the cell-membrane; and this change, 

 originally consequent on a physico-chemical action, may be 

 so maintained and utilized by natural selection, as to result 

 in structures of definite kinds, regularly formed in growing 

 parts in anticipation of functions to be afterwards discharged. 

 But, without alleging any special cause for this metamor- 

 phosis, we may reasonably conclude that it is in some way 

 consequent upon the carrying of sap. If we examine tissues 

 such as that in the interior of a growing turnip that has not 

 yet become stringy, we may, in the first place, find bundles 

 of elongated cells not having yet developed in them those 

 fenestrated or reticulated structures by which the ducts are 

 eventually characterized. Along the centres of adjacent 

 bundles we may find incomplete lines of such cells — some 

 that are partially or wholly transformed, with some between 

 them that are not transformed. In other bundles, completed 

 chains of such transformed cells are visible. And then, in 

 still older bundles, there are several complete chains running 

 side by side. All which facts imply a metamorphosis of the 



