394 



THE FORMS OF TISSUES 



[CH. 



the space to be cut off be a very small one, not more than about 

 three-tenths the area of a square based on the short side of the 

 original rectangular cell. As the portion usually cut off is not 

 nearly so small as this, we get the form of partition shewn in 



Fig. 178. Diagrammatic development of Stomata in Sedum. (Cf. fig. in 

 Sachs's Botany, 1882, p. 103.) 



Fig. 179, and the cell so cut off is next bisected by a partition at 

 right angles to the first; this latter partition splits, and the two 

 last-formed cells constitute the so-called "guard-cells" of the 

 stoma. In other cases, as in Fig. 178, there will come a point 

 where the minimal partition necessary to cut off the required 

 fraction of the cell-content is no longer a transverse one, but is 

 a portion of a cylindrical wall (2) cutting off one corner of the 



mother-cell. The cell so cut off 

 is now a certain segment of a 

 circle, with an arc of approxi- 

 mately 120° : and its next division 

 will be by means of a curved wall 

 cutting it into a triangular and 

 a quadrangular portion (3). The 

 triangular portion will continue to 

 divide in a similar way (4, 5), 



>- 



Fig. 179. Diagrammatic development ^nd at length (for a reason which 

 of stomata in Hyacinth. is not yet clear) the partition wall 



