94 



BOTANY 



I'AllT 1 



wall, remains alive. The walls of sieve-tubes are always unliguitied. 

 Their sap-cavities contain a watery, and more or less dilute, solution 

 of albuminous substances, and small starch grains are also present 

 in most cases. As a rule, the sieve-tubes remain functional only 

 through one or a few vegetative periods. Before their activity 

 ceases the sieve-plates become covered (Fig. 103 C) with highly 

 refractive callus (p. 69). 



A more complete fusion of protoplasts is found in the formation 

 of laticiferous vessels. These have the same structure and contents 



- 



~" 



FIG. 101. A cell from the cortex of the 

 Mistletoe (Viseum album) ; the protoplast 

 has been properly fixed and stained and 

 the wall (m) swollen. The pit-membranes 

 (x) are traversed by connecting threads ; 

 ck, chloroplasts ; n, nucleus, (x 1000.) 



, - . 



i 

 pi 



' 



. 





B 



Flo. 10:.'. A, A swollen portion of cell wall 

 from tin- endosperm ol tlie Vegetable Ivory 

 Palm (1'hyteli ill/a* //m.-,v,r,./y><i). At t, s, 

 simple pits tilled with cytoplasm ; in the 

 intervening pit-membrane are fine connect- 

 ing threads ; pi, other threads traversing 

 the whole thickness ol the wall, (x 375.) 

 li, The contents of two opposed pits and 

 the connecting threads of the pit -membrane. 

 (X 1500). C, The opening of a pit ami tin- 

 connecting threads ot the pit-membrane 

 viewed from the surface, (x 1500.) 



as latex cells (p. 72). Their occurrence, like that of latex cells, is 

 limited to a few distinct plant families, such as the Papaveraceae, of 

 which the Poppy (Paparer) or Celandine (Chelidonium), with its 

 characteristic orange-coloured "sap," are familiar examples, or the 

 Composited, of which in particular the Lettuce (Lactuca) may be 

 cited. Latex vessels are distinguished from latex cells only by the 

 method of their development, which has resulted from the fusion of 

 rows of elongated cells, the separating transverse walls of which 

 have become more or less completely absorbed. Such vessels as a 

 rule possess lateral branches, which, on meeting with one another, 

 fuse by the absorption of their end walls (Fig. 104). The 

 mucilage tubes ( 61 ) which are found in many Monocotyledons 

 present points of resemblance to the laticiferous vessels. Their 



