•SECT. I 



MORPHOLOGY 



79 



consequent separation from the cell wall is brought about (see plasniolysis). Such 

 protoplasts are able under certain conditions to surround themselves with a new 

 cell-membrane. The removal of the cuticle or of the w^axy covering from the 

 surface of certain plants [Agave, Aloe, Ricinus, Sedum), is followed by its re- 

 generation C^"). 



Form of the Cell. — As cytoplasm is a viscous fluid, and would tend, 

 if unimpeded, to take a spherical shape, it may be assumed that the 

 natural and primary form of cells is 

 spherical. Such a shape, however, could 

 only be realised by cells which, in their 

 living condition, were completely free and 

 unconfined, or in such as were able to ex- 

 pand freely in all directions. Newly de- 

 veloped cells, in a continuous tissue, are, at 

 first, nearly always polygonal. Through 

 subsequent growth their shape may change. 

 The cubical cells of the growing point, with 

 a mean diameter of 1-2 fjL, either elongate 

 to a prism or remain, owing to repeated 

 division, short and tabular. If the growth 

 is limited to certain regularly arranged 

 points of the surface, they become stellate ; 

 if these points are less uniformly arranged 

 their outline is correspondingly unsym- 

 metrical. In consequence of energetic 

 growth in length, fibre-like, pointed cells 

 are developed. If the walls of such cells 

 become much thickened, they are called 

 SCLERENCHYMA fibres (Fig. 85 A). These 

 show diagonal markings, due to their elong- 

 ated pits, which are generally but few in 

 number. When fully developed, the living 

 contents of such cells are small in amount 

 and frequently they contain only air. In 

 the last case, they merely act as mechanical 

 elements (stereides) and contribute to the 

 rigidity of the plant as a whole. Cells 

 somewhat similar, but shorter and considerably wider, not pointed at 

 the ends, and provided with bordered pits, are called tracheides (Fig. 

 85 ^). The tracheides, in their fully -developed condition, never have 

 any living contents, but serve as water-carriers for the plant. So long 

 as they remain active, they contain only water and isolated air-bubbles ; 

 their active functions afterwards cease, and they become filled with air. 

 Tracheides which are specially elongated, and at the same time have 

 only a narrow lumen, and, like the sclerenchymatous fibres, serve merely 

 mechanical purposes, are known as fibre tracheides. Very long 



Fig. 85. — A, A sclereiicliymatous 

 fibre ; B, a tracheide ; C, jiart of a 

 spiral traclieide ; D, part of a latex 

 tube. (A, B, C, X 100; D, x 

 circa 150.) 



