ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 289 



Connecting Threads of Protoplasm in Pinus.* — Arthur W. Hill has 

 investigated the nature and distribution of the connecting threads in the 

 various tissues of the hypocotyl, cotyledons, and root of the young 

 seedling of Pinus pinea, and of the adult stem, leaf, and root of P. syl- 

 vestris. 



Connecting threads were formed in the case of all cells in which the 

 wall retains its cellulose or mucilaginous character, and in all such 

 young tissues as the growing point of the root. In P. pinea, the tissue 

 of the endosperm and that of the germinating seedling are well con- 

 nected by such threads. In the cotyledon the walls of the cells, both of 

 the epiderm and of the subjacent parenchyme, are more richly provided 

 with threads than the similar tissues of the upper side. In the stomates 

 of the cotyledon threads were seen in a few cases connecting the guard- 

 cells with the epidermal cells. The parenchymatous tissue throughout 

 the seedling shows connecting threads of a similar character. The living 

 cells of the pericycle are abuudantly connected by long usually curved 

 threads. The root-cap shows numerous threads connecting its cells 

 together, associated, no doubt, with its function as an organ for the per- 

 ception of stimulus. In P. sylvestris the characters of the threads in the 

 cortical tissues of the adult stem and root are similar to those of the 

 seedling. In the phloem no threads can be found directly connecting 

 the parenchymatous cells with the sieve-tubes. The existence of threads 

 in the xylem is doubtful. The leaf of P. sylvestris shows a distribution 

 of connecting threads similar to that in the cotyledons. 



Centrifugal and Simultaneous Thickenings of the Membrane.f — 

 Having shown the probability that the centrifugal thickenings of the 

 membrane in Peridiniefe, diatoms, and desmids is due to the activity of 

 an extra-cellular layer of protoplasm, F. Schiitt now shows that in many 

 cases where a centrifugal thickening of the membrane is prima facie 

 assumed, there are no subsequent changes in form or bulk when the 

 portions of the membrane in question have once been formed ; that the 

 whole growth takes place simultaneously ; and that there is no question 

 of thickening. This is especially the case with the spines and rods of 

 the plankton-diatoms, which often combine the separate individuals into 

 long chains. A favourable example is presented by the genus Sceletonema, 

 where tufts of rods from the valves which face one another connect the 

 two frustules with one another. After the division of the mother-cell, 

 the two naked protoplasts of the daughter-cells separate from one 

 another, then again come into contact, and again completely separate, 

 being connected only at certain points, where subsequently the rods are 

 in contact with one another; and, as they retreat from one another, these 

 rods are spun out from the protoplasmic layer. 



It is seldom, according to the author, that an actual centrifugal 

 growth of the membrane takes place, but this does occur in the case of 

 the teeth on the spines of Chaetoceras. The extracellular needles of 

 Cyclotella socialis are spun out from the external layer of protoplasm. 

 The chambers in many diatom membranes are also probably formed 

 simultaneously. 



* Proc. Roy. Soc, lxvii. (1901) pp. 437-9. Cf. this Journal, 1900, p. 476. 

 t Pringsheim's Jahrb. f. wiss. Bot., xxxv. (1900) pp. 470-534 (1 ph). Cf. this 

 Journal, 19U0, p. 74. 



