154 VITALITY AFFECTED BY POISONS. [BOOK n. 



the acid be very weak, the leaflets dilate and appear to live, 

 but are insensible. A drop of the acid placed on two leaflets 

 of a healthy plant gradually causes contraction of the other 

 leaflets, pair by pair. Solutions of opium and corrosive 

 poisons have no effect when applied this way. After some 

 time they dilate, but are insensible to external irritation : the 

 sensibility returns in about half an hour; but the leaflets 

 appear as if benumbed. 



The plant exposed to the vapour of prussic acid is affected 

 in the same way. Ammonia appears to favour the recovery 

 of the plant. 



A cup containing dilute prussic acid was so placed that 

 one or two leaves, or sometimes a branch, of a healthy plant 

 could be plunged into the liquid, or left to repose on its sur- 

 face. The leaflets remained fresh and extended, but were 

 almost immediately insensible. Being left in this state for 

 two hours they were expanded ; and no irritation could cause 

 their contraction, though otherwise there was no appearance 

 of an unnatural state. At five o'clock in the evening the 

 leaves were left to themselves. At nine o'clock they were 

 open and insensible. At midnight they were still open, 

 whilst all the rest of the plant, and the neighbouring plants, 

 were depressed, contracted, and in the state of sleep. On 

 the morrow they resumed a little sensibility, but seemed 

 benumbed. 



In the same manner Macaire interfered with other plants 

 as to the state of sleep, and found that prussic acid thoroughly 

 deranges the botanical indications of time of Linnaeus. 



It is, however, conceivable that phenomena may occur 

 among plants, having at first sight all the appearance of vital 

 actions, but in reality produced by mere physical causes. 

 Of this nature appears to be the catalepsy of Physostegia 

 virginiana, described by Professor Morren. 



The inflorescence of this plant is a close spike. The flowers 

 are opposite in pairs, in a decussate manner, and are about 

 three-quarters of the length of a calyx from each other. 

 The phenomenon consists in this, that if you turn a flower 

 standing in face of you so far to the right or left as to stand 



