WHAT IS A PLANT? 77 



through the water. In this state, they closely resemble many of 

 the Infusoria, a group of Protozoan animals, including the so- 

 called " animalcules." Examples of these are seen in the motile 

 forms of Frotococcus, residing in our water-butts and roof-gutters ; 

 in Vaucheria^ ColeochcBte, Chlajtiydamonas, and Volvox globator, 

 the " Globe Animalcule " ; notably, in Pero7iospo7'a infestajis^ the 

 fungus causing potato-disease, which, both in the development of 

 cilia and in the use it makes of them, bears decided resemblance 

 to an Infusorian. Form, therefore, supplies us with no distin- 

 guishing character on either side. 



II. — Presence of Cellulose. Microscopically speaking, the 

 structure of animals and plants consists of multitudes of minute 

 hollow bodies, termed cells, seen in diverse forms. It was for- 

 merly held that the enveloping membrane, or cell-wall, of these 

 cavities was in animals composed of gelatine, in plants of cellu- 

 lose (CcHioOs). A distinction was accordingly founded on this 

 presence of cellulose in plants. It is now known that this sub- 

 stance exists abundantly in the outer tunic, or test, of the Asci- 

 dioida^ or " Sea-Squirts," a molluscoid group of animals, standing, 

 according to modern systematic zoology, next below Vertebrata. 

 Archer, of Dublin, found it not long since in the self-secreted case 

 of Chlamidoniyxis^ a fresh-water Protozoon.* Cienkowski, a Russian 

 naturalist, discovered it in the inner wall of the capsule of Vaui- 

 pyrella.\ This organism is seen in the shape of minute brick-red 

 capsules attached to the filaments of the well-known water-weed 

 Spirogyra, with whose chlorophyll it literally gorges itself, perforat- 

 ing cell after cell of the plant, and abstracting their green contents. 

 In all these cases, the presence of cellulose is proved by the cha- 

 racteristic blue reaction under the action of iodine and sulphuric 

 acid. 



This distinction, therefore, breaks down. So constant a 

 feature, however, of plant-life is this cellulose wall, that we may 

 say that, morphologically, the most distinctive feature of plant-life 

 is the presence of a cell-wall containing cellulose ; that of animal 

 life its absence. 



* Journal Lhui. Soc, Zoology, Vol. XIIL, p. 281. 

 t Ibid, p. 419. 



