FORMATION OF THE COMMON WALL OF CELLS. 
75 
succulent fruits (e.g. Symphoricarpus in winter), and can sometimes be artificially 
brought about by continued boiling in water (as in potato-tubers). 
The origin of the partition- walls in tissue- cells which increase by bipartition 
by no means requires the supposition that they were originally composed of two 
lamellae. In this case one would be led, by a consideration of the properdes 
of tissues where numerous divisions follow one another and intercellular spaces 
afterwards arise, to extrernely complicated hypotheses, which, moreover, are not 
in harmony with growth by intussusception. Even where the union of the cells 
into a tissue arises from the amalgamation of originally separate cells (not sister- 
cells), the union of the cell-walls is so intimate that no boundary-line can any 
longer be perceived; and the formation of a middle lamella proves also in such 
cases ^ (as does the formation of a middle lamella generally) that the hypothetical 
boundary-surface does not exist, and that the splitdng of the homogeneous lamella 
is a consequence of different growth on its two sides. Both the manner in which 
the splittings of the thin homogeneous partition-walls arise, and also the formation 
of the middle lamella of thick walls, contradict the supposition of an originally 
double partition-wall in tissue-cells ^. 
The splitting of the pardtion-wall and the growth of its now separated lamellae 
lead to a variety of configurations in the interior of tissues, which may be col- 
lecdvely included in the conception of the Intercellular Space. To this belong 
especially the large air-conducting channels in the tissue of many w^ater and marsh- 
plants (Nymphseaceae, Irideae, Marsileaceae, &c.), and the formation of the cavity 
between the wall and the spore-sac in the sporogonium of Mosses ^ Peculiar 
processes of growth of the adjoining cells are not unfrequently connected with 
the origin of intercellular spaces, of which the following are examples: — the for- 
mation of ordinary stomata, of the peculiar stomata of the Marchantieae, and of 
resin and gum-passages i^ide infra). 
But the behaviour of the partition-wall of two cells contributes in quite a dif- 
ferent manner to the production of air- or sap- conducting channels, which, like 
the air- or sap- conducting intercellular spaces, may form a condnuous system 
throughout the plant. This happens by the partial or entire absorption of the 
partition-walls of adjoining cells, by which the cavides of long rows of cells of a 
tissue become connected. Unger has appropriately designated this a Coalescence 
of Cells. Vessels of this kind {Trache'ides of Sanio) are formed in the xylem 
of the fibro-vascular bundles, from which the protoplasm and cell-sap have dis- 
appeared ; they serve for conducting air. In the sieve-tubes in the bast-portion 
of the fibro-vascular bundles, on the other hand, the watery mucilaginous 
contents of the cells are not replaced by air ; the communicadon established 
^ For examples see Hofmeister, Handbuch, vol. I. pp. 262, 263. 
^ I may remind the reader of the cleavage of crystals as an analogous case ; the cleavage- 
surfaces are determined by the molecular structure, but there is a wide difference between them 
and true fissures, however fine. 
^ The wide air-canals in the stem of Equisetacese, Grasses, species of Allium, Umbelliferre, and 
Compositse, arise, on the other hand, from the cessation of the growth of inner masses of tissue and 
their drying and splitting, while the surrounding tissues continue to grow. 
