FORMATION OF THE COMMON WALL OF CELLS. 



73 



flesh of many succulent fruits {e. g. in the snowberry in winter) ; and this separation 

 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 careful consideration of the properties 

 of those tissues where numerous divisions follow one another and intercellular 

 spaces afterwards arise, to extremely complicated hypotheses (which, moreover, also 

 contradict growth by intussusception). Even in those cases where the union of 

 the cells into a tissue arises from the amalgamation of originally separate cells 

 (which are 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 splitting of the 

 homogeneous lamella is a consequence of different growth on its two sides. Both 

 the manner in which the splittings of the homogeneous thin partition-walls arise, and 

 also the formation of the middle lamella of thick walls, oppose the supposition of an 

 originally double partition-wall in tissue-cells ^. 



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

 lead to a variety of configurations in the interior of tissues which may be col- 

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

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

 plants (Nymphaeaceae, Irideae, Marsileaceae, &c.), and the formation of the cavity 

 between the wall of the capsule and the spore-sac in the fruit of Mosses ^ Not 

 unfrequently peculiar processes of growth of the adjoining cells unite in the origin 

 of intercellular spaces. I will here only allude to three very diff'erent examples, 

 the formation of stomata, the air-cavities of INIarchantia, and resin and gum pas- 

 sages {vide infra). 



But in quite a diff'erent manner the behaviour of the partition-wall of two cells 

 contributes to the production of air- or sap-conducting channels, which, like the 

 air- or sap-conducting intercellular spaces, may form a continuous system in the col- 

 lective mass of the substance of a 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 ; and the single cells themselves 

 become members of a bag-like or tubular structure. Unger has appropriately 

 designated this Coalescence of Cells. Vessels of this kind (TracheVdes of Sanio) are 

 formed in the wood of fibro-vascular bundles, in w^hich the protoplasm and cell-sap 

 disappear, and they serve for conducting air. In the sieve-tubes in the bast- 

 substance of the fibro-vascular bundles, on the other hand, the watery mucilaginous 



^ For examples, see Hofmeister, Handbuch, I. pp. 262, 263. 



"^ Further detail of this subject is not possible here. I may here remind the reader only 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. 



3 The wide air-canals in the stem of Equiseta, Grasses, species of Allium, Umbelliferoe, and 

 Compositse arise, on the other hand, from the cessation of the growth of inner masses of tissue and 

 their drying and bursting, while the surrounding tissues continue to grow. 



