54 



BOTANY 



PAKT I 



111 



which their whole central portion is filled by a single, large sap 

 cavity {v in B, Fig. 57). The cj'toplasm then forms only a thin layer 



lining the cell wall, while the nucleus occupies 

 a parietal position in the peripheral cyto- 

 plasmic 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 posi- 

 tion the nucleus may occupy, it is always 

 embedded in cytoplasm ; and there is always 

 a continuous peripheral layer of cytoplasm 

 lining the cell wall. This cytoplasmic peri- 

 l^heral layer is in contact 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 per- 

 ceptible until some reagent, which attracts 

 water, causes it to recede from the wall, has 

 been emploj^ed. 



Dead cells lose their living proto})lasmic 

 contents, and, strictly speaking, should no 

 longer be termed cells, although the name 

 ^cy was first applied to them when in that condi- 

 tion. In reality they represent only cell 

 cavities. With their death, however, cells do 

 not lose their importance to a plant. With- 

 out such cell cavities a highly-organised plant 

 could not exist, as they perform for it the 

 office of Avater-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- 

 plasm, attention Avill first be directed to the 

 Slime Fungi (Myxomycetes), a group of 

 organisms which stand on the border between 

 the animal and vegetable kingdoms. These 

 Myxomycetes are characterised at one stage 

 of their development by the formation of a 

 PLASMODIUM, a large naked mass of ])roto- 

 2)lasm. 



The Plasmodium is formed from the protoplasm of the spores. 

 These spores arc vmicellular bodies (Fig. 58 a, h), filled with cytoplasm, 



Fio. 57.— Two culls takijii at 

 diHerent distances from tlie 

 growing \mmt of a plianero- 

 tjainic .shoot, k, Nucleus ; cy, 

 cytoplasm ; c, vacuoles, re- 

 jircsonted in B by the sap 

 cavity. (.Somewhat diagram- 

 matic, X circa .000.) 



