FORM OF THE PLANT-CELL, 



67 



66 



extend through the entire length of a plant, and often end in a cul de sac. 

 This is so obvious in some forms of leafless Euphorbiacece, that it is 

 wonderful how any difference of opinion could have existed on the point. 

 In the older vessels, also seen well in the leafless EuphorbiacecB, the 

 spiral bands and the deposit-layers on the walls are easily distinguished, 

 so that the lateral development of these organs agrees entirely with that 

 of cells. 



In their relation to one another, these three forms of tissue, as well as 

 the receptacles of milk without proper walls, seem mutually to represent 

 each other. They are seen before the vascular bundles of the stem, as 

 receptacles of milk in Mammillaria, as liber in Cereus, as transitionary 

 forms in the Apocynacece and Asclepiadacece, which in some species 

 resemble liber-cells, whilst in others, as in Sarcostemma viminale, they 

 are not to be distinguished from milk-vessels. 



History and Criticism. The liber and the milk vessels were known 

 to the earliest observers. The proper walls of the last were first seen 

 by Mirbel, but more accurately observed by Schultz, whose observa- 

 tions, overloaded by false theory and hasty inferences, have led to this 

 principal result, that a large proportion of the milk -vessels do really 

 possess a peculiar covering.* His theory of their origin, founded on 

 insufficient observation, has become now quite antiquated. linger 



Natur der lebenden 



* Ueber Circulation des Saftes im Schollkraut. Berlin, 1821. 

 Pflanze. Berlin, 1832. 



66 A, A transverse section of a thickened milk -vessel from the bark of the stem of 

 Euphorbia ccerulescens, with the walls of the same lying upon the cells. B, A longi- 

 tudinal section of the same, isolated through maceration. The sides of the vessel are 

 irregular, from being pressed into the surrounding cells. 



67 Longitudinal section from the rind of Euphorbia trigona, parallel to the medullary 

 rays. Many of the vessels branch arid anastomose, whilst others end in blind extremi- 

 ties. Irregularly formed starch granules are seen in their interior. 



F 2 



