BOTANY 



large sap cavity (v in , Fig. 58). The cytoplasm then forms only 

 a thin layer lining the cell wall, while the nucleus takes a parietal 

 position in the peripheral cytoplasmic layer. 

 At other times, however, the sap cavity of 

 a fully-developed cell may be traversed by 

 bands and threads of cytoplasm ; and in that 

 case the nucleus is suspended in the centre of 

 the cell. But whatever position the nucleus 

 may occupy, it is always embedded in cyto- 

 plasm ; and there is always a continuous peri- 

 pheral layer of cytoplasm lining the cell wall. 

 This cytoplasmic peripheral layer is in con- 

 tact with the cell wall at all points, and, so 

 long as the cell remains living, it continues 

 in that condition. In old cells, however, it 

 frequently becomes so thin as to escape 

 direct observation, and is not perceptible 

 until some dehydrating reagent, which causes 

 it to recede from the wall, has been employed. 

 Such a thin cytoplasmic peripheral layer has 

 been described by HUGO v. MOHL under the 

 name of PRIMORDIAL UTRICLE. 



Dead cells lose their living protoplasmic 

 contents, and, strictly speaking, should no 

 longer be termed cells, although the name 

 was first applied to them when in that, con- 

 dition. In reality they represent only cell 

 cavities. With their death, however, cells 

 do not lose their importance to a plant. 

 Without such cell cavities a highly-organised 

 plant could not exist, as they perform for 

 it the office of water-carriers, and afford 

 mechanical support and rigidity. The heart 

 wood of a tree consists exclusively of the 

 walls of dead cells. 



The Protoplasm. In order to facilitate 

 . an insight into the real character of proto- 



Fio. 08. Two cells taken at J , 



different distances from the plasm, attention will first be directed to the 



growing point of a phanero- SLIME FUNGI (Myxomycetes),a grOUpof Orgail- 

 gamic shoot. k. Nucleus ;. i_-i_ A.-UJUJ it. 



!*, cytoplasm; v , vacuoles, 1SmS whlch stand On the b . rder between the 



represented in B by the sap animal and vegetable kingdoms. These 

 cavity. (Somewhat diagram. Myxomvcetes are characterised at one stage 



niatic, x circa 500.) , J . . J , , , .1 t t 



of their development by the formation of a 

 PLASMODIUM, a large naked mass of protoplasm. 



The plasmodium is formed from the protoplasm of the spores. 

 These spores are unicellular bodies (Fig. 59 a, fc), filled with cytoplasm, 



