FORMS AND SYSTEMS OF TISSUES. 



81 



FIG. 68.— Transverse section of a part of the petiole of Nuphar advena ; 

 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- 

 manniese, or even the entire plant, as among Algae in Vl'va and Rytiphlcea. 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 elastic a. 



(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 Floridese, 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 aquilina 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 Ghroococcaceae, 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 diiferent 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 leaf of Psoralea hirta; 

 w. a group of laticiferous cells imbedded in the chlorophyll- 

 tissue/ ; e e the upper and under epidermis. 



