MORPHOLOGY 99 



boiling soft tissues in water, the cells may often be easily isolated through the 

 consequent swelling and dissolutipn of the middle lamella. In ripe fruit, an 

 isolation of the cells frequently takes place spontaneously, through the dissolution 

 of the middle lamella. A lignified middle lamella, on the other hand, seems able 

 to withstand more effectually the action of oxidising agents. Consequently, it 

 is possible, by subjecting a section of pine -wood to the action of SCHULZE'S 

 MACEKATING MIXTURE (potassium chlorate and nitric acid), and subsequently 

 treating with concentrated sulphuric acid, to remove all secondary and tertiary 

 thickening layers, so that only the middle lamellae remain as a delicate network. 

 If the macerating process be continued for a longer time, without the subsequent 

 treatment with sulphuric acid, the middle lamellae become finally dissolved. The 

 thickening layer will then be left free from all lignification. SCHULZE'S macerating 

 method may accordingly be employed to isolate the elements of lignified tissues. 

 The peculiar relation of the middle lamella towards chemical reagents gave rise 

 at one time to the presumption of a special intercellular substance which, like a 

 glue, bound together the cells of a vegetable tissue. The supplementary deposition 

 of pectic compounds in the middle lamellae (p. 69) frequently gives rise to the 

 formation of rod -like protuberances and excrescences, which project into the 

 intercellular spaces, or these spaces may be filled up by the formation of gussets 

 (Fig. 70 C, m*). 



Kinds of Tissue. The mature tissues of the plant may be divided 

 into two groups, PARENCHYMA and PROSENCHYMA, between which, how- 

 ever, no sharp distinction can be made. A typically developed paren- 

 chymatous tissue is one in which the thin-walled cells are equally 

 expanded in all directions, and possess a layer of protoplasm lining 

 the wall, and other cell contents. Typical prosenchymatous tissue, 

 on the other hand, consists of thick-walled, elongated cells, either in 

 the form of fibres or spindle-shaped cells, with interlocking, pointed 

 ends, and with little or no protoplasmic contents. A parenchymatous 

 tissue, in which the cells are thick-walled and elongated, resembles 

 prosenchyma, but may be distinguished from it by the absence of 

 pointed cell terminations, and especially by the greater abundance 

 of protoplasm. Thin- walled prosenchyma is not, on the other hand, 

 necessarily lacking in protoplasm, but is characterised by its pointed 

 and interlocking cells. 



An undifferentiated tissue, the cells of which are still capable of 

 division, is termed embryonic tissue, or MERISTEM. The meristem of 

 embryonic rudiments and of the growing point is called PROMERISTEM, 

 and all meristematic tissue which can be shown to have been derived 

 directly from such promeristem is termed PRIMARY. A primary 

 meristem may still retain its meristematic character in the midst 

 of a mature tissue. Fully differentiated tissue is designated 

 PERMANENT tissue, or somatic tissue, in contrast to meristematic tissue. 

 At times, permanent tissue may again become capable of division, and 

 thus give rise to a SECONDARY MERISTEM. 



A mass of tissue so united in the body of a plant as to form 

 a higher histological unit constitutes a tissue system. In the more 



