DEVELOPMENT OF CELL-WALLS 61 



The new cell- walls formed in connection with cell-divisions are at 

 first thin, unstratified and entirely homogeneous membranes; at this 

 stage, in fact, they should he regarded as partitions separating adjacent 

 protoplasts rather than as integral portions of individual cells. At a 

 somewhat later period each wall usually consists of at least three 

 obviously distinct layers, namely, a central middle lamella, overlain by 

 cellulose layers belonging to and laid down by the protoplasts on either 

 side of the wall. The middle lamella therefore constitutes a septum 

 separating the cellulose membranes of two adjacent cells ; it was at one 

 time in fact regarded as an intercellular substance. Very often the 

 structure described as the middle lamella merely represents the primary 

 partition-wall, which has undergone a certain amount of chemical 

 modification since its formation ; more often, however, it also com- 

 prises the primary thickening layers. In all cases the middle lamella 

 differs chemically from the secondary and tertiary thickening layers. In 

 soft tissues it consists, according to Mangin, of a pectic compound 

 of calcium. In ripe fruits this pectic substance often undergoes 

 mucilaginous degeneration, with the result that the cells become 

 separated from one another ; a similar gelatinisation may be artificially 

 effected by boiling the tissues. In the case of lignified and suberised 

 tissues the middle lamellae are often more or less strongly lignified, 

 and further characterised by their insolubility in sulphuric acid. 



A striking contrast to the production of tissues by means of 

 repeated cell-division is furnished by the secondary concrescence of 

 originally distinct cells. This mode of tissue-formation is, however, 

 comparatively rare. A familiar instance is provided by the develop- 

 ment of the disc-shaped coenobium of Pediastrum. Here any cell of 

 the disc may divide to form a number of zoospores, which emerge from 

 their mother-cell still enclosed in a thin vesicle representing the 

 innermost layer of the mother-cell membrane. The zoospores at first 

 exhibit active swarming movements, but soon come to rest, and arrange 

 themselves in a single plane, whereupon they acquire cell- walls, become 

 attached to one another, and thus give rise to a small daughter-disc. 

 While it is customary to describe the disc of Pediastrum as a coeno- 

 bium or colony of unicellular individuals, the fact remains that these 

 individuals are firmly knit together to form an entity of a higher order 

 of magnitude ; from this point of view a Pediast 'rum-disc must be 

 regarded as a tissue produced by concrescence. 



The above-described two extreme types of tissue-formation 

 namely, repeated cell-division and concrescence of originally separate 

 cells are connected by a number of intermediate forms, some of the 

 most important of which may next be briefly discussed. 



