CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. loy 



being mostly limited to very minute membranous scales. They are usually easily 

 distinguished by their origin, position, and by the production of leaves, from the 

 leaf-tendrils ; cases, however, occur where the morphological nature of a tendril 

 is doubtful, as, for instance, in Cucurbitaceae. Peculiarly clear examples of stem- 

 tendrils are to be met with in Vitis, Ampelopsis, and Passiflora. Shoots which 

 bear strongly developed foliage-leaves on long slender internodes, and which have 

 the power of winding in an ascending manner round upright supports, are not 

 considered tendrils, but are called Twining or Climbing Sterns^ \ and thus a dis- 

 tinction is drawn between Tendril-climbers (as Vitis) and Stem-chmbers (as Phase- 

 olus, Humulus, Convolvulus, &c.). In Cuscuta, where the primary shoot and all 

 the lateral shoots except the inflorescences twine in the manner of tendrils and 

 of climbing stems, and where foliage -leaves are also entirely suppressed, the 

 peculiarities of tendrils and of climbing stems are to a certain extent united. 

 A distinction similar to that between stem -tendrils and climbing stems is also 

 possible in leaves ; the foliage-leaves of the Fern-genus Lygodium, endowed with 

 a continuous power of growth in length, behave completely like climbing stems, 

 the climbing rachis of the leaf corresponding to a climbing axis, and the leaflets 

 to its foliage-leaves ^. 



The axial shoots of many Angiosperms have, like the leaves, the power ol 

 forming Spines, becoming transformed into conical, pointed, hardened bodies, 

 This may take place either by the whole shoot or even a whole system of shoots 

 becoming spiny, with suppression of the foliage-leaves, as in the branched spines 

 of Gkditschia ferox, or by the shoot first producing foliage-leaves, growing in the 

 ordinary manner, and finally finishing its growth in length by a spiny point, as 

 in the lower axillary shoots of Glcditschia Iriacan/hos, Primus spitiosa, and man) 

 others. 



Among Phanerogams, especially among Monocotyledons and Dicotyledons, relative 

 positions of the leaves and lateral shoots (as well as of roots), and mutual adhesions ol 

 members, constantly occur, which, as development advances or becomes mature, are ir 

 apparent contradiction to the typical laws of growth and position, /. e. to those which are 

 the ordinary ones in these classes. It would be difficult even for a thoughtful and 

 clever beginner to explain by the principles which have been regarded in this chaptei 

 as most universal, the structure, for instance, of an expanded flower of an Orchis 

 rose, Lamium, Salvia, and of many other plants, the structure of a partially or wholl) 

 ripe fig, or the phyllotaxis in the inflorescences of Asperifolieae and Solanaceae, anc 

 many others. But the history of development shows that even such cases may be 

 ranged under these laws ; and that the peculiarities of structures of this kind only arise 

 during a later period of development, or in such a manner that they confirm genera! 

 rules. The deviations from these rules are caused by the cessation of the growth oJ 

 particular parts at an early period in their development, while others undergo a great 

 advance ; or they are caused by the adhesion of parts originally distinct. Although it is 

 quite impossible to give general rules for the explanation of irregular formations, yet 



1 Compare H. von Mohl, Ueber den Bau und das Winden der Ranken und Schlingpflanzen 

 Tiibingen 1827. [See also Uaiwin, On the Movements and Habits of Climbing Plants, Journ 

 Linn. Soc. vol. IX.] 



2 Compare Book II, Ferns, and Book III, on the Physiological Signification of Tendrils anc 

 climbing Stems. 



