132 



LECTURE VIII. 



consists only of septate filaments, or even only of isolated cells, as in many Algae, 

 we still find at the surface, at least cuticularised outer layers of the cell-wall, or 

 even an actual cuticle-; and this is not wanting even on the isolated cells of vascular 

 plants, e.g. spores, and pollen-grains, — and indeed is often strongly developed on them. 

 To the characteristic peculiarities of the epidermis, especially of vascular plants, 

 belong the structures known as hairs. In the narrower sense of the word, hairs 

 are outgrowths of individual epidermis-cells, which however, by means of their 

 growth and physiological adaptations, may assume the most various forms. According 

 to circumstances, these outgrowths of the epidermis cell are wart-like projections, 

 vesicles, elongated tubes, shield-like outspread scales, prickly outgrowths, or soft 



FIG. 123.— Development of the hairs on the calyx of a flower-bud of AUhaa rosea ( x 300) ; vjh (in A) 

 woolly hairs on the inner surface ; b and c glandular hairs in various stages of development ; o. (to the right) 

 rudiment of a glandular hair; ep everywhere, the (still young) epidermis. The figures a in ^, j3 (to the 

 left) and -y (to the right, lower figure) first stages of development of the stellate (or rather tufted) hairs, of 

 which later stages may be compared in Fig. 126. In At o. shows the hair in longitudinal section ; ^ and V 

 seen from above. The cells are rich in protoplasm ; in Y the development of vacuoles (tj) is beginning. 



woolly coverings, and so forth. Moreover, these outgrowths may remain simple 

 tubes, or, as they grow further, become transformed by means of more or less 

 numerous transverse and longitudinal divisions into multicellular organs. Among the 

 vascular plants, there are only few, as the aerial shoots of Equisetums, Conifers, and 

 Duckweeds {Lemnacea), where the formation of hairs generally is wanting. Usually, 

 indeed, one and the same plant is furnished with various kinds of hairs, often so 

 densely that hardly anything else is to be seen on the surface of the organ. 



There is scarcely any other organ in the vegetable kingdom where the two 

 different principles — on the one hand the mere formative force of organic substance, 

 and on the other hand the adaptation to definite life-purposes — come so clearlj' into 



