238 Dr. Schleiden on the Anatomico-physiological 



cus, &c., in short all thick formative saps. At the period 

 when the development of these cells ceases, they are called 

 by Mohl vasa 'propria. From this cause all further deve- 

 lopment of these vascular bundles is rendered impossible, 

 and therefore I call them terminated^ or " limited.^^ In Dico- 

 tyledons, on the contrary, this tissue, which is then termed 

 cambium^ Auct., couche regeneratrice^ Mirb., retains, during 

 the whole lifetime of the parts of the plant, its vital forma- 

 tive power ; it continues to develope new cells, and by means 

 of them increases the mass, as they go on adding partly 

 to the exterior portion {liber), and partly to the interior 

 (wood), to infinity. This happens according to the climate 

 and nature of the plant, either pretty continuously, as for 

 instance in the Cactece"^, or by strong periodical advances 

 alternating with almost entire cessations, as in our forest trees. 

 In the latter, one may be convinced by perseverance and 

 delicate manipulation, that the stem forms a continuous tis- 

 sue, from the pith to the bark, in all periods of its life, and 

 that the bark is never separated from the stem ; what has 

 been so considered is only a rent produced by manipulation 

 of the delicate formative tissue, which is in a great measure 

 present, even during the winter, constituting the foundation 

 of the new annual zones, although compressed, and filled 

 with gum, starch, &c. In the spring, being expanded and 

 swollen by the new current of sap, it is deprived of its con- 

 tents by their solution. In all cases w^e may convince ourselves 

 that the new cellular tissue is always formed within that 

 already existing, and, in fact, in primitive cells, by means of 

 cytoblasts, in the same manner as I have already previously de- 

 monstrated with regard to other cells. Indeed, the young cells 

 are constantly formed on the upper or lower (I regret that I 

 have not yet paid sufficient attention to this point,) end of the 

 elongated primitive cells, and by means of their expansion 

 lengthwise grow through them, and their contact with the 

 other end of the cell appears to call into existence a new cell 



* For this reason the observation of the whole process is most easy in the 

 Cactuses. In general they have also remissions of growth, which do not 

 altogether correspond with the annual impulses, although they produce 

 similar appearances. The cause is yet wholly unknown. 



