DIAPHRAGMS. 219 



especially at the margin, often with relatively very large cavities at the corners (comp. 

 Meyen, Physiol. I, Taf. II, Figs. 3 & 4). The greater part of the surface of these 

 diaphragms has, on the other hand, the following structure as a rule. The surface ap- 

 pears divided into polygonal areas, and each of these is again divided by parallel walls 

 (which may be termed the inner walls) into 4 cells on the average, of which the central 

 ones are narrow and quadrangular, the outer are irregular and narrow, with 3-5 corners. 

 The inner walls of an area are usually not parallel to those of the neighbouring areas. 

 The walls which limit the areas, and apparently correspond to those of the mother cells 

 which are subsequently divided by the inner walls, are irregularly undulated and un- 

 interruptedly connected with those adjoining them. But along the parallel inner walls 

 each of the cells has a row of usually 5-7 short arms, and between these are roundish 

 quadrangular cavities. 



The several-layered lacunar diaphragms naturally resemble in the main points the 

 masses of stellate-lacunar parenchyma above mentioned. Those only of the halm of 

 Papyrus, which run transversely, though slanting and distorted, through many air- 

 passages, deserve special mention, on the one hand, because of the extremely irregular 

 form and arrangement of the arms of their cells, on the other, because their lacunar 

 tissue is also continued transversely through the lateral walls of the air-passages, from 

 one to the other, and from the outermost into the peripheral chlorophyll-parenchyma. 

 Since all diaphragms, or at least most of them, are continued transversely through 

 several or many air-passages, all the passages communicate, by this arrangement, in- 

 directly one with another, with the air-containing interstitial spaces of the chlorophyll- 

 parenchyma, and through this with the stomata, though in the lateral walls of the 

 passages themselves no interstitial spaces are to be found. 



In such plants as secrete much calcium oxalate, not only is it often laid up 

 in large quantity in the form of small crystals in the cells which adjoin the air- 

 passages, as e.g. in the diaphragms of Musa and Sagittaria, but the structure of 

 the walls, both laterally and on the diaphragms, is often complicated by the interr 

 calation of crystal-containing sacs in the layer covering the walls, or these are seated 

 upon the above layer as papillae or small hairs. As far as is known this only occurs 

 in schizogenetic, not in lysigenetic spaces : it remains for observations of the develop- 

 ment to decide whether Nelumbium, with its numerous grouped crystals, which 

 protrude into the air-spaces, is an exception to this. Of the forms of crystal- 

 containing sacs described in Sect. 32, we have here to deal more especially with 

 elongated or spindle-shaped sacs with raphides, and spherical sacs, each one en- 

 closing a single stellate group of crystals. 



Such of these as are intercalated in the layer covering the walls require no 

 further mention here. The sacs with clustered crystals which protrude into the 

 cavity are always seated on the wall, singly or (Trapa) in groups, as small round 

 bladders with a broad base. When old their membrane, which is always delicate, is 

 in many cases extremely thin and difficult to see — it remains doubtful whether it 

 entirely disappears or not— so that the clusters project or appear to project freely 

 into the cavity. 



The projecting, elongated or spindle-shaped raphide-cells are sometimes attached 

 to the lateral walls, in which case they either have one of their ends inserted on it, 

 or are attached by the middle to a narrow surface of a cell of the wall, while the two 

 ends project freely upwards and downwards into the space. The Same holds some- 

 times for the one-layered diaphragms, and the walls of the chambers (also one layer 

 of cells thick) in the leaf of Pistia; sometimes in this plant the raphide-cells have 



