l6o MORPHOLOGY OF MEMBERS, 



growth that the internodes expand in a plane which also includes the axis of length, and 

 thus become leaf-like, as in Ruscus, Xylophylla, &c. 



In leaves the principal growth is usually in a plane which cuts the stem transversely, 

 and is mostly symmetrical right and left of a plane which includes the axes of length 

 both of the leaf and the stem ; the common form of leaves is therefore that of thin 

 plates symmetrically divided in half in the direction of their length. There occur, 

 however, cylindrical and roundish tuber-like leaves, in which the growth has been nearly 

 equally rapid in all diameters at right angles to the axis of the leaf, as in Mesembryan- 

 themum ech'matum ^. 



Sect. 22. Hair (Trichome) '^ is the term given in the higher plants to 

 those outgrowths which arise only from the epidermis, i.e. from the layer of cells 

 which always remains the outermost in roots, stems, and leaves, whether these 

 outgrowths assume the form of simple tubular protuberances, rows or plates of cells, 

 or masses of tissue, or have the physiological character of woolly envelopes of the 

 young leaves, root-like absorbing organs as in Muscinese, glands, prickles, or spo- 

 rangia as in Ferns ^. 



Hairs may originate from the primary meristem of the growing point, or from 

 young leaves and lateral shoots, if an external layer of cells has already been 

 differentiated as dermatogen, as in Phanerogams; but they may originate also in 

 much older parts the tissue-systems of which have already become further differ- 

 entiated, and which exhibit interstitial growth, because in such cases the epidermis 

 produces new cells, for example stomata, and long remains capable of cell- division. 



When hairs spring from the growing point, they are usually formed after the 

 leaves, i. e. further from the apex than the youngest leaves ; but it also occurs in 

 Phanerogams that they are developed above the youngest leaves and nearer to the 

 apex, the outermost layer of cells of the growing point having in this case already 

 become differentiated as dermatogen, as in Utricularia according to Pringsheim. 

 In Muscinese and Vascular Cryptogams also, where the leaves become visible long 

 before the differentiation of the external layers of tissue, the hairs do not appear on 

 the surface of the stem till a later period and further from the apex. 



If the hairs arise near the apex of a growing point or on a zone of interstitial 

 basal growth, as do the sporangia of Hymenophyllacese, they may be arranged 

 according to a definite law, which is not the case with hairs that spring from older 

 organs, or at least not evidently so. 



Hairs are always strikingly different in their form from the leaves and lateral 

 shoots of the same plant, although they sometimes bear a certain resemblance to 

 these organs in other plants. The development in size of a single hair is usually 

 extremely small compared to that of the member which produces it ; even the mass 

 of all the hairs of a leaf, root, or stem is generally quite inconsiderable compared 

 to the weight of the organ. 



^ [The leaf is also frequently unsym metrical, i.e. the growth has not been equally vigorous 

 of the two halves separated by the axial plane, as in the lime, Begonia, &c.] 



2 Rauter, Zur Entwickelungsgeschichte einiger Trichomgebilde. Vienna 18 71, p.. 33. — Compare 

 also Sects. 15 and 19 (b). — Warming, Sur la difference entre les trichomes et les epiblastemes, d'un 

 ordre plus eleve (extract from the Videnskabelige Meddelelser de la societe d'Hist. Nat. de 

 Copenhague, nos. 10-12, 1872. 



3 [Hairs may develope into adventitious buds, as in Begonia; see Caruel, Trasformazione di 

 peli in gemme, Nuov. Giorn. Bot. Ital,, July 1875.] 



