45(3 SUMMARY OF CURRENT RESEARCHES RELATING TO 



which may not take placo until after the formation of the basal portion. 

 To this there are some striking exceptions in the unlimited apical growth 

 of the leaves of Nephrolepis and of some Gleicheniacere. 



The CycadcsD exhibit the same variation in this respect as ferns. 

 Among Conifera), on the other hand, apical growth appears always to 

 cease at a very early period. 



In Monocotyledons the apical growth of the leaves is in general very 

 limited, the intercalary growth being of far greater importance. 



Among Dicotyledons three distinguished types of leaf-growth may 

 be distinguished, viz.: — (1) Intercalary, where the lateral segments, 

 whether pinnaB or teeth, proceed from a point which is not at the apex 

 of the leaf; the apex soon passes into a resting condition, the growing 

 point lying below it ; (2) apical, where all the lateral segments of the 

 first order proceed from the growing apex ; and (3) an intermediate type, 

 partaking of the characters of the other two. To the first type belong 

 the leaves of the greater number of herbaceous and woody Dicotyledons. 

 The lateral segments may arise in either basipetal or basifugal succession. 

 Of the second type the most striking examples are the Umbelliferae, most 

 Legurninosse (except the Acaciere and CaBsalpiniese), and some Filices 

 and Cycadese. To the third type belong the leaves of a large number of 

 Composites. 



Influence of External Forces on the Form of Plants.* — Herr F. Noll 

 discusses the question, What is the source of the energy which deter- 

 mines the varying growth and development of the different parts of a 

 plant ? If we take such a unicellular organism as Caulerpa and Bryopsis, 

 the different parts of the single cell are differentiated physiologically, 

 but not anatomically, since it is possible, by reversing the position, at 

 once to convert the apex of the " stem " into a " root." There can be 

 here no protoplasm peculiar to stem, leaf, or root, since the protoplasm, 

 with its chromatophores and nuclei, is in constant motion from one organ 

 to another, and cannot therefore determine the difference in the nature 

 of the irritability of the different organs. This power must reside in a 

 substance which remains permanently attached to each organ ; and, since 

 it cannot be referred to the cell-wall, it must belong to the quiescent 

 parietal layer of protoplasm, which must be the seat of the properties of 

 geotropism and heliotropism. The same argument applies also to the 

 cells of the higher plants, where the granular protoplasm is in constant 

 varying circulation or rotation, the parietal utricle alone remaining at 

 rest. The continuity of protoplasm which has been demonstrated from 

 cell to cell is a continuity of this active parietal homogeneous, not 

 granular, protoplasm. 



Transpiration as a Function of Living Protoplasm.f — Eev. G. 

 Henslow gives the results of numerous experiments, from which he draws 

 the following general conclusions : — That plants which do not possess 

 chlorophyll at all transpire more under light than in darkness, but ex- 

 hibit slight, but not very appreciable, differences under light of various 

 colours. Transpiration appears to be more sensitive to increments of 

 temperature than to colours, and perhaps than to pure white light itself. 

 Etiolated plants still show some slight difference due to coloured rays. 



* Versamml. Deutscb. Naturf. u. Aerzte, Wiesbaden, September 20, 1887. See 

 Bot. Centralbl., xxxiii. (1888) p. 29. 



t Joum. Linn. Soc. Lond. — Bot., xxiv. (1888) pp. 286-307. 



