DIV. I MORPHOLOGY 71 



substances, and accumulates between the outer wall of the secretory 

 cells and the cuticle which is raised up and finally burst. The same 

 holds for the formation of other adhesive substances and mucilage. 



According to the excreted products, which may have varied ecological uses, 

 the epidermal glands may be distinguished into mucilage, oil, resin, digestive 

 (Fig. 77) glands, also salt glands, water glands (hydathodes), and nectaries ( 48 ). 

 The last-named secrete a sugary fluid which attracts insects and occur as 

 glandular surfaces or hairs within the flower or in other situations (cf. Fig. 136 ). 

 These are termed respectively floral and extra-floral nectaries. 



The glandular cells or epithelia enclosed within parenchymatous or 

 other tissues always abut on circular or irregular intercellular spaces or 

 tubular, branched, or unbranched canals which sometimes run through 

 the whole plant as a* connected system of tubes. These intercellular 

 spaces, which arise by the splitting apart of cells, form the schizogenous 

 secretory reservoirs (Fig. 78). Their contents consist of ethereal oils, 

 resin, gum, or mucilage, and corresponding distinctions are made in 

 naming these canals. 



Schizolysigenous reservoirs also occur. 



SECTION III 



ORGANOGRAPHY ( 49 ) 



THE EXTERNAL MEMBERS AS ORGANS OF THE PLANT 



THE organisms included in the vegetable kingdom are variously shaped 

 and segmented. Some are unicellular throughout life, while others 

 are multicellular. Both may have very simple and regular geometrical 

 forms and have no external segmentation, or on the other hand may 

 possess a body with a very irregular outline owing to its being 

 divided into protrusions of the most various kinds. 



I. Significance of the External Segmentation to the Organism. 

 The construction and segmentation of any particular organism stand 



as a rule in close relation to its needs and mode of life. The external 

 as well as the internal segmentation is usually the expression of a 

 DIVISION OF LABOUR between the parts or the cells of the multicellular 

 body. The external members are, in fact, usually ORGANS with definite 

 vital functions. The physiological progression from simpler to more 

 segmented organic forms consists in great part in the increase of this 

 division of labour. 



II. Main Groups of Organs. The activity of every organism has 

 two sides. It must nourish itself in order to maintain itself as an 

 individual, and it must reproduce in order that the race should not 

 perish with its death. The body of the plant subserves these two 

 fundamental vital impulses. Only in primitive plants does the whole 



