OF GROWTH. 465 



must distinguish three several processes, namely, growth in the 

 narrow and literal sense, that is, the formation of new cells ; the 

 unfolding or extension and enlargement of cells already present ; 

 and the lignification or thickening of the cell-walls by spiral and 

 porous layers. Each of the three contributes in different ways to 

 the perfecting of the entire plant and its organs. It is especially 

 important to distinguish the first and second of them precisely. 

 The process termed germination is marked by two periods, the 

 first of which consists in the softening and extension of cells al- 

 ready formed, and the second in the formation of new cells. The 

 rapid growth of the seta of Jungermannia belongs to the process 

 of unfolding, also the extension of the internodes in the Phanero- 

 gamic plants. But here we greatly need exact and comprehensive 

 investigations. 



Essential growth goes on, so far as is at present made out, by 

 the formation of new cells in the interior of the old or parent 

 cells, which, by resorption, set the new ones free. No other mode 

 of increase of the cells has been as yet fully established. 



I have already asserted*, and I believe I have made it clear, that 

 there can be no scientific treatment of the vitality of plants without an 

 accurate distinction between the three above-mentioned phenomena, and 

 in every case an apprehension of which of the three is actually present. 

 This is so simple, that when once attention is called to it, it will be 

 understood, for examples of the three kinds of growth must be known to 

 every botanist. 



By attention to the first and second division, we obtain a distinction 

 of two essentially different periods in the development of every part of 

 a plant ; first, the period during which the cells which constitute its 

 substance are formed ; and secondly, when they become expanded. The 

 two periods are often very accurately separated from each other, as, for 

 example, with many petals ; in other cases the one passes into the other, 

 as in the anthers. 



In botanical books a number of examples are given of periodical acce- 

 lerations or retardations of growth.f All these examples are useless for 

 the derivation of laws, because the previous distinctions have not been 

 at all attended to. Treviranus, for example, quotes the rapid repro- 

 duction of the anthers in an ear of rye, when they have been stripped 

 off by passing through the mouth. In this case it is only a question of 

 the distinction of previously existing cells ; the same also with respect 

 to the development of the flower-stalk of the Agave. Thus the 

 investigation of E. Meyer on barley and wheat (Linnaea, vol. iv.), and 

 of Mulder on the leaves of the Urania speciosa (Bydragen tot de 

 naturk. Wetensch. vol. iv.), with respect to the comparative growth by 

 night and by day, and at the varying hours of the day, are useless, be- 

 cause no distinction has been made between the formation, and the mere 

 expansion of the cells. To this branch of the subject properly belongs 

 .ill that has been said upon the distinction in the growth of the stem, 

 the root, the leaves, and all other parts (see Treviranus, Physiologic, 

 vol. ii. pp. 152 179.). All experiments and observations that have 



* M tiller's Archiv, 1838, p. 158. ; Beitrage zur Botanik, vol. i. p. 141. 

 f TVeviranus, Physiologic, vol. ii. p. 442. 



H H 



