1883.] development of the Leaves of Pinus Silvestris L. 351 



the nucleus, which unites two of the apparent cellulose infoldings, 

 and slings the nucleus firmly to one wall. Where the nuclei in two 

 neighbouring cells are connected in the early stages by filaments 

 of protoplasm to the same point of the common partition-wall 

 between them it comes to pass that in the process of enlargement 

 of the cells, when the walls are become thickened, this wall being 

 strained in two opposite and contrary directions splits along its 

 centre and there results at a slightly later period two opposite 

 pseudo-ingrowths of cellulose, often leaving an intercellular space 

 between them 1 . To the irregular form which the cells assume in 

 consequence of their unequal enlargement, owing to parts being 

 held in, the intercellular spaces which are met with in the tissue 

 of the adult leaf are due ; for in the early stages as has been 

 already mentioned they are absent. The nuclei in these cells are 

 never free, and in this feature they present a marked contrast to 

 what occurs in the case of the colourless medullary parenchyma 

 where the protoplasm disappears early and the large circular 

 nuclei are nearly always found lying freely in the cell cavity. The 

 only explanation which has been hitherto advanced as to the 

 nature of the so-called "infoldings" which form such a characteristic 

 feature of the adult leaf and have long been known to exist there 

 is that given by Prof. Sachs, who considers that " on the partial 

 splitting of the partition wall a local growth of one or both of the 

 two lamellae (or of one only) takes place so that a fold arises 

 which intrudes into the cell cavity 2 ." This view is based partly 

 upon the appearance presented in section by the cells of the adult 

 leaf and partly upon the analogy of certain other cases there 

 quoted; it is not however as I have shown supported by the results 

 obtained from a study of the facts of development, and is contrary 

 to the usual mode of growth and expansion of the cell. The so-called 

 " intrusive folds or ingrowths " are not at all of this nature, for no 

 intrusion or real invagination ever takes place ; they simply and 

 only occur at the points where the protoplasmic filaments of the 

 nuclei in the original columnar cells were connected with the walls, 

 and where in the subsequent enlargement of the cell-wall retarda- 

 tion of growth has occurred. How the appearance of intrusion 

 comes about I have already explained. Some of the simplest and 

 least complicated cases may always be seen in the walls of the 



1 I feel sure that the so-called "pit-like formation" figured by Sachs (toe. cit. 

 fig. 60) is uot of this nature, for I have never observed any appearance of pits of 

 this kind, but is simply due to two abortive pseudo-ingrowths on a longitudinal 

 radial wall, which having caused a splitting of the common partition, and the form- 

 ation of two shallow bays, have then ceased to be connected with the nuclei, owing 

 to the breaking away of the protoplasmic filaments which united them to it; and 1 

 have been able to observe every intermediate stage between these so-called "pit-like 

 formations" and characteristic pseudo-ingrowths. 



- Text-Book of Botany, 2nd English Edition, 1882, p. 74. 



