ORGANIC FORM AND ARCHITECTURE 699 



canals of most Conifers are the semi-normal results of strain 



and stress. 



Many of the cells in animals retain throughout life their power of 

 dividing. Thus as the outermost layer of the epidermis or "scarf 

 skin" in higher animals is worn away, it is replaced by fresh growth 

 from beneath ; to put it technically, the worn-out stratum comcum 

 is always being renovated by contributions from the stratum 

 mucosum. The same kind of replacement is common in other parts 

 of the body; but there are some cells, notably those of the brain. 

 which do not multiply after birth. 



In plants there is much multiplication of cells in parts that are 

 still growing, but in contrast to most animal types there is a frequent 

 persistence of embryonic tissue at strategic points. This tnerisUm, 

 as it is called, is well illustrated by the growing point of a stem or 

 of a root, and by the cambium-layer which forms a ring between the 

 inner wood and the outer bast in a Dicotyledonous stem. This 

 local persistence of non-differentiated ever-young cells is very 

 characteristic of plants; but there are also many cases of division 

 of cells in relatively differentiated permanent tissue. 



When cells remain rich in protoplasm, thin-walled, well expanded 

 in all directions, the term parenchyma is used, in contrast to pros- 

 enchyma, in which the cells are elongated, thick-walled, pointed at 

 the ends, interlocked, and not usually rich in protoplasm. Thus 

 there is a deep contrast between parenchymatous and prosenchy- 

 matous tissues. 



All that we wish for our purpose here is an outline survey of the 

 chief plant-tissues, and these include : 



(i) the skin in the wide sense — the tegumentary tissues; 



(2) the fibro-vascular bundles — beginning to be prominent in 



Ferns and their relatives; 



(3) the fundamental tissue system, mostly parenchymatous, 



e.g. in the substance of leaf and shoot and root. 



(i) The most important tegumentary tissue is the epidermis, 

 usually a single layer of colourless cells, often with the outer walls 

 thickened into a cuticle, and bearing a great variety of superficial 

 structures. When these are entirely epidermic they are known as 

 "hairs", like the glandular papillae on the Chinese Primrose or the 

 stings of nettles; but when sub-epidermal tissues help they are 

 conveniently called "emergences", as in the prickles of the rose and 

 the glandular tentacles of sundews. Often there are glandular 

 specialisarionsin the epidermis, such as groups of cells (hydathodes), 

 which secrete watery fluid, or nectaries which secrete sugar, or the 

 pitcher-plant's httle pockets which secrete digestive juice. A 

 noteworthy feature of epidermic cells is the intcriocking of their 

 cell-walls, which probably reduces the risk of lesions under strain. 



