Observations show however that the statement of Sachs needs modi- 

 fication. Twenty to twenty- five etiolated leaves have been examined, 

 with this result: The stronger etiolated leaves become hollow; only 

 the weaker ones remain solid. 



The different structural relation in the normal , hollow leaves 

 and in the etiolated leaves with no cavity or with a small cavity are 

 not difficult to trace. In the very young leaf, even down to the bulb 

 near its origin, the middle mass of parenchyma contains large inter- 

 cellular spaces, while the peripheral cells are very small and the hori- 

 zontal radial and tangential diameters are equal. As the young leaf 

 elongates and increases in thickness, the intercellular spaces in the 

 middle tissue increase greatly in size , the cells separating so as to 

 form a network. As the leaf grows up into the light, the peripheral 

 cells enlarge greatly in every direction, forming the well-known 

 H- palisade assimilating cells and large intercellular spaces. The 

 middle parenchyma does not grow so rapidly, and the strain breaks 

 the network. The middle cells are however living at the time and 

 live for some time after they are split apart. The cells that have the 

 smallest surface connection with the neighboring tissue are the first 

 to die. 



In the etiolated plants the peripheral cells had grown but little 

 in size and had preserved the form which they had within the bulb. 

 Thus there was no inflation here as in the normal leaves , but in 

 cross-section the leaves had the form of an ellipse or meniscus. In 

 the larger leaves however the growth of the periphery was too great 

 for that of the central parenchyma, and some splits took place followed 

 by collapse of cells. Where only a few centimeters of a leaf are 

 kept in the dark , the etiolated part will grow enough to form a 

 cavity. 



The formation of a cavity in a short segment of a leaf may be 

 prevented by the application of gypsum casts, though here of course 

 etiolation as well as pressure is effected. There are two difficulties 

 however connected with the successful use of gypsum. In the first 

 place the leaf of Allium is so smooth that it will grow out of the 

 cast. In the second place the meniscus-shape of the cross-section of 

 the leaf at the time when the cast must be applied usually allows 

 the infolding of the flat side as the pressure by growth increases. 

 The result of this infolding of the strong periphery is the stretching 

 and crushing of the weak parenchyma in the middle. In 10 leaves 

 around which casts of 3 cm length were kept for 5 weeks, the flat 

 side had in each case folded in, and each leaf had a mass of dead 

 cells in the middle, though the number of rows of living hyaline 



