FURTHER DEVELOPMENT OF WELWITSCHIA MIRABILIS. 591 
crystals; these are represented in fig. xviit as dark dots. They 
extend from the outer cellulose wall to the cell cavity, and are 
filled with plugs of protoplasm (fig. xtx), which are continuous 
as far as the outer cellulose wall. The protoplasmic contents 
become more and more reduced as the development of the ligni- 
fied walls proceeds, till they appear to be finally lost. 
The mode of development of the spicular cells may be com- 
pared with that of the internal hairs in the intercellular spaces 
of the Nymphzeacez, and in the tissues of certain Aroidee (cf. 
De Bary, ‘ Vergl. Anat.,’ p. 230, &c., where the literature is 
cited). The spicular cells are an interesting intermediate between 
these and sclerenchyma fibres, which they resemble in many 
respects. 
Sieve Tubes. 
The sieve tubes of Welwitschia have already been described 
by Strasburger (‘Coniferen,’ p. 380), and figured by Bertrand 
(“Cellules grillagées,” ‘Ann. d. Sci. Nat.,’ série v, vol. xx). 
But in longitudinal sections from the root I have obtained differ- 
ent results from the latter writer. I find the sieve tubes to 
resemble those of the other Gymnosperms in their general cha- 
racters, but the sieve plates are collected almost exclusively on 
their sloping ends (fig. xxt). Occasionally one is met with on 
the lateral walls. The sieve plates are often coloured a sherry- 
brown with Schultz solution, while the rest of the walls is blue. 
The contents are transparent and almost free from granules. 
Between the sieve tubes appear cells of the bast parenchyma. 
Results. 
1. The cotyledons wither and fall off, and the pair of leaves of 
the mature plant are the first pair of plumular leaves. 
2. The crown is derived entirely from the continued growth 
of two lobes (axillary buds), which appear in the axils of the 
cotyledons. ‘The apical cone of the plant remains rudimentary. 
3. The parenchymatous “ground tissue” throughout the 
plant remains, for a long period at least, capable of active growth 
nd division (halbmeristematisch). The results of this are (a) 
increase in bulk of the tissue itself; (4) production of fresh 
spicular cells, and sclerenchyma fibres by the growth of single 
cells of it; (¢) formation of fresh vascular bundles by means 
of repeated divisions at certain points. 
N.B.—It should here be observed that the active tissue has a 
different origin in different parts of the plant; in the root it is 
derived from the “tissu conjunctif” of Van Tieghem, in the 
stem from the fundamental tissue generally. 
