FORMS AND SFSTEMS OF TISSUES. 
8i 
FIG. 68.— Transverse section of a part of the petiole of Nnphar m'i'ciia; 
i large intercellular spaces bounded by simple layers of cells ; s s stellate id oblasts ; 
g a fibro-vascular bundle. 
rare for a whole organ to consist of a single layer of cells, as the leaves of Junger- 
mannieae, or even the entire plant, as among Algae in Ulnja and Rytiphlaa. In the 
higher plants the epidermis usually consists of a single layer, and this is not unfre- 
quently the composition of the vascular bundle-sheath (always in the case of young 
roots) ; in water and marsh- 
plants the fundamental tissue 
often resolves itself into simple 
layers which enclose large in- 
tercellular spaces, as in Nuphar, 
Fig. 68, Sal'vinia, Musa, &c. 
It is not uncommon for masses 
of tissue to be composed of a 
number of simple layers of 
cells, as occurs frequently in 
the secondary wood of trees 
and the primary cortex of 
branches and roots ; or seve- 
ral simple layers alike among 
themselves are in close juxta- 
position, as in the epidermis 
of the leaves of Begonia and 
Ficus elastica. 
(4) A String or Bundle 
of Cells is an elongated mass 
of tissue, the transverse sec- 
tion of which consists of a 
number of cells. Many of 
the lower plants, such as some simple Fungi and Florideae, consist of such strings 
of cells. The fundamental tissue of the higher plants is sometimes traversed by 
bundles of peculiar cells ; such are the 
brown sclerenchymatous strings in the 
stem of Pteris aquilma and of Tree-ferns. 
The true bast of Dicotyledons not unfre- 
quently forms bundles in the soft bast. 
In all vascular plants the bast-like and 
wood-forming elements are united into 
bundles, the Fibro-vascular Bundles, which 
form true tissue-systems, and traverse the 
fundamental tissue. 
(5) Groups of Cells are roundish aggre- 
gations of similar cells. In the lower Algae, 
as the Chroococcaceae, groups of this nature 
arise, each from a single mother-cell, and 
carry on an independent life as Cell-families. 
In the fundamental tissue of the higher 
plants groups of peculiar cells are often 
formed, strikingly different from those that 
surround them, as for example the groups 
of laticiferous cells illustrated in Fig. 69, 
or the groups of sclerenchymatous cells in 
the soft flesh of pears. True (compound) 
glands are formed by the dissolution of such groups of cells {'vide infra). 
(6) In the cases already named a number of similar cells are always united into 
a whole; but it also frequently happens that a single cell acquires a character 
G 
Fig. 69. — Vertical section of a lea"" of Psoyalca hirta; 
ni a group of laticiferous cells imbedded in the chlorophyll- 
tissue p \ e e the upper and under epidermis. 
