109 



is easily discernible. In many Cerei, Opunticc, and Peireskitz 

 the number of the thorns in each bundle increases with age, 

 new ones annually shooting forth from the centre. In some, 

 especially in the Echinocacti, there is frequently a central thorn 

 which seems to terminate the growth ; and here the number of 

 thorns is almost universally constant as soon as the plant is 

 full-grown. The thorns are not always arranged in circles in 

 the bundles, but in the Echinocacti with sharp edges stand in 

 the form of an elongated ellipse, at whose upper end gradually 

 the last thorns and lastly the flowers make their appearance. 

 The flowers and new shoots of wood almost always appear 

 above the bundle of thorns, and are consequently not encircled 

 by them, which is even the case in Mammillaria prolifera. 

 The thorn bundles of the Mammillaria usually throw forth 

 neither flowers nor branches, and consequently, with the ex- 

 ception of a few species (Mammillaria vivipara,parvimamma) 

 which sprout from the mammillae but do not flower, are con- 

 stantly abortive ; while in the Mammillarice the flowers pro- 

 ceed from the stem close above the pulvinus. Here therefore 

 sterile or abortive buds and wood- and flower-buds occur on 

 the stem, and M. Zuccarini sho\rs that this double formation 

 of buds is also known in other plants. 



M. J. Wttewaall has published a very elaborate memoir on 

 the nature of the stem in a physiological and morphological 

 point of view*. It is divided into four chapters: the first 

 treats of the development of the annual stem, the second of 

 the further growth of the stem, the third of the subterranean, 

 and the fourth of the difference between root and stem : they 

 are all written with a great knowledge of the subject and of 

 its literature, and will be of great value to subsequent mor- 

 phologists. In the consideration of the development of the 

 annual stem, the author sets out from a view, which enjoys 

 great favour also with us in Germany, viz. that of the compo- 

 sition of the cauliculus from the cohering petioles of the coty- 

 ledons, a notion which is completely refuted by actual ob- 

 servation of the development of those separate parts. It may 

 indeed be replied, that^I ought to give the proof: however, I 



* Jets over het onstaan, den groei, en de vormveranderingen van den 

 stengel. Tijdscrift voor natuurlijke geschiedenis en physiologic, Vierde 

 Deel, 2 e Steek, 1837, p. 42105. 



