The Cellas a Member of a Group. == 47 
- # 
The special characteristic of ce//-fustons consists in the 
cells of which they are composed uniting completely by 
coalescence into a uniform whole of a higher type. Unions 
of this kind are usually accompanied by the transformation 
and absorption of at least a portion of the surfaces of con- 
tact. Fusions of a less complete kind have already been 
described in the coalescence of the contiguous cells of ‘ tela 
contexta ;’ unions of a more perfect description occur in- 
conjugation and the formation of vessels. 
The process of conjugation, to which reference has- 
already been made, begins with the lateral coalescence of 
two contiguous cells, and the formation of an opening at the 
point of contact; finally the contents of the two cells 
coalesce in the production of a spore (Fig. 54, p. 32). This 
process has hitherto been observed only among Fungi and | 
Algee, and especially in the family of Conjugatee belonging — 
to the latter. 
Vessels arise from the coalescence of several cells, which 
are then termed vascular cells ; the division-walls_ partially 
disappearing while the union continues at the margin. 
Vessels are divided, according to their forin and contents, 
_ into vessels in the true sense of the word, bast-tubes or 
bast-fibres, sieve-tubes or bast-vessels, vesicular vessels, and 
laticiferous vessels. 
If a number of spiral cells are imagined placed one over __ 
another and coalescing from the more or less complete ab- 
sorption of the partition-walls, a spzral vessel is produced. 
Vessels are formed in a precisely similar manner, from several 
‘of the kinds of cells previously described ; and we thus get 
annular vessels, reticulate vessels, scalariform vessels (Fig. 69)," 
and pitted or dotted vessels (Fig. 70). 
The individual vascular cells sometimes stand vertically one over an- 
other, and coalesce by their horizontal faces ; sometimes they are in con- 
tact laterally, and with faces that stand obliquely. The partition-walls 
do not always entirely disappear ; but often, especially when they stand | 
obliquely, portions remain which are more strongly thickened and of a 
