DIAPHRAGMS. 217 



Sect. 52. During the formation of longitudinal air-passages, whether they are 

 formed simply schizogenetically or by destruction of cells, transverse zones, which 

 follow the extension, remain connected at certain places, as diaphragms, which break 

 the continuity of the passage. 



In the first place, they occur in the nodes of all hollow stems, and here they are 

 deep many-layered discs of dense parenchyma, through which vascular bundles, milk- 

 tubes, and secretory passages run, and pass out into the leaves, as will be described 

 in later chapters. In the internodes the large, simple, axile cavity of the hollow stem 

 is uninterrupted, at all events in most cases, and the same holds for the numerous 

 peripheral passages in the internodes of many water-plants, as Ceratophyllum, 

 Myriophyllum, Hippuris, Elatine Alsinastrum, species of Jussisea, Limnanthemum 

 nymphoides, Zostera, Posidonia Caulini, Nelumbium : also, with a restriction to be 

 given below, for the leaf- and fiower-atalks of the native Nymph^aceae : and lastly, 

 for all roots with large air-passages, though these require further investigation in this 

 respect. On the other hand the air-passages in the internodes, petioles, and leaves 

 of most Monocotyledons, the petioles of Limnanthemum nymphoides, the internodes 

 and petioles or conical leaves of the Marsiliacese, the leaves of the Isoeteae, &c., are 

 partitioned by diaphragms. 



These are separated by short distances, usually a few millimetres, rarely over 

 i"™, from one another; they are horizontal or oblique; they may alternate, those of 

 adjoining passages being at unequal heights, or they may be at about the same height, 

 so that one and the same diaphragm extends evenly over several or many passages. 



The diaphragms consist of one, more rarely of several layers of parenchyma- 

 tous cells, often rich in chlorophyll, between which air-containing interstitial spaces 

 always lead from one chamber into another : they are sometimes composed of dense 

 parenchyma with narrow interstitial spaces, as e. g. in the leaves of Luzula maxima 

 which has diaphragms two layers thick, in species of Carex, Cladium Mariscus, 

 Scirpus sylvaticus, maritimus, Cyperus fuscus, Veratrum album. Iris pseudacorus, 

 Posidonia Caulini, Zostera, Caladium, Colocasia and its allies, &c. ; others consist of 

 many-armed cells, connected by the ends of the arms, and forming a plate with wide 

 lacunae, as in the leaves and stems of many water-plants, Isoetes, Potamogeton, 

 Aponogeton, species of Typha, Sparganium, Pontederia, Butoraus, Sagittaria, and 

 Alisma, petioles of Limnanthemum, Strelitzia, halms of Papyrus, Heleocharis palus- 

 tris, Eriophorum, leaves of Pandanus, &c. In the wide central portion of the halm 

 of Juncus effusus, glaucus, and their allies, transverse zones of some few layers of 

 many-armed stellate cells keep their walls relatively firm, and persist as diaphragms, 

 while the delicate tissue between them, which is also many-armed and lacunar, 

 collapses. 



Intermediate forms, which make it impossible to separate the lacunar dia- 

 phragms sharply from the dense ones, are not uncommon: for instance, those 

 composed of short-armed cells, with narrow cavities between, in Scirpus lacustris, in 

 the leaf-sheaths and leaves of Glyceria aquatica, and Oryza sativa (Duval- Jouve), to 

 which many of those above-named correspond ; and the narrow-lacunar diaphragms 

 of the Marsiliacese, the structure of which is not unlike that of the lateral walls of the 

 air-passages, &c. 



Two sorts of diaphragms were mentioned above in the case of the air-passages 



