FORMATION OF THE COMMON WALL OF CELLS. yt 



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 properties 

 of tissues where numerous divisions follow one another and intercellular spaces 

 ifterwards arise, to extremely complicated hypotheses, which, moreover, are not 

 (n harmony with growth by intussusception. Even where the union of the cells 

 (nto a tissue arises from the amalgamation of originally separate cells (not sister- 

 :ells), the union of the cell-walls is so intimate that no boundary-line can any 

 mger be perceived; and the formation of a middle lamella proves also in such 

 ises ^ (as does the formation of a middle lamella generally) that the hypothetical 

 )undary-surface does not exist, and that the splitting of the homogeneous lamella 

 a consequence of different growth on its two sides. Both the manner in which 

 le splittings of the thin homogeneous partition-walls arise, and also the formation 

 )f the middle lamella of thick walls, contradict the supposition of an originally 

 louble partition-wall in tissue-cells ^. 



The splitting of the partition-wall and the growth of its now separated lamellae 

 [ead to a variety of configurations in the interior of tissues, which may be col- 

 jctively included in the conception of the Intercellular Space. To this belong 

 jspecially the large air-conducting channels in the tissue of many water and marsh- 

 "plants (Nymphaeaceae, Iridese, Marsileacese, &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 origii* 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 {vide 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 continuous system 

 throughout the plant. This happens by the partial or entire absorption of the 

 partition-walls of adjoining cells, by which the cavities 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 communication 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, Umbelliferse, and 

 Composite, 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. 



