216 ABSORBING SYSTEM 



water economy of the Bryophyta, and is also widely distributed among 

 Phanerogams. It is necessary, however, to distinguish between two 

 degrees of specialisation in this respect. The leaves of a great many 

 plants are able to absorb water when partly withered, simply because 

 the cutinisation of the outer epidermal walls does not render them 

 absolutely impervious. In such cases one is dealing with a purely 

 incidental property, which may occasionally redound to the advantage 

 of the plant, but which cannot in any way affect the normal balance of 

 its water-interchange. Many epiphytes on the other hand especially 

 Bromeliaceae and also certain steppe- and desert-plants, depend for 

 their water-supply entirely upon the ability of their leaves to absorb 

 the moisture that becomes available in the form of rain or dew. In 

 these cases, accordingly, the aerial portions of the plant are provided 

 with special absorbing organs, which not infrequently exhibit a com- 

 plicated and highly adaptive structure. 



Among epiphytic Orchidaceae and Aroideae, again, the aerial 

 roots possess, in the so-called "velamen," a special apparatus which 

 serves for the absorption of dew and rain, and perhaps also for the 

 condensation of water-vapour. 



The case of Fungi and other non-chlorophyllous plants must next be 

 considered. Owing to the absence of chloroplasts, such plants must lead 

 a saprophytic or parasitic existence, or, in other words, are entirely 

 dependent upon organic food-materials. Here also, absorption is carried 

 on over the whole external surface in the simplest types, such as Yeasts 

 and Bacteria. At a higher stage of evolution, division of labour leads 

 to the segregation of the propagative organs from a purely vegetative 

 region of the plant-body, the task of absorption being then entrusted 

 to the latter, which in the most specialised types becomes entirely 

 converted into a ramified absorbing organ. The filamentous mycelium, 

 which is so characteristic of the Fungi, serves mainly as such an 

 organ of absorption ; in the case of certain Phanerogamic parasites the 

 vegetative body becomes likewise reduced to a system of haustorial 

 organs. 



In the matter of absorption, embryos and also seedlings, so long as 

 they are dependent upon the mother-plant for their supplies of plastic 

 material occupy a position which is in a sense intermediate between 

 the condition of green plants and that of non-chlorophyllous forms. 

 In these instances, also, the office of absorption is, among the more 

 specialised types, entrusted to special haustorial organs ; if the latter 

 are bulky multicellular structures, they are generally furnished with 

 a definite absorbing tissue. 



The preceding general remarks will have made it evident that 

 the structure of the absorbing system gives a clear indication of the 



