LVCOPODIACEM. 



407 



these genera in appearance is Phylloglossum, a small Australian plant only a few 

 centimetres in height, which puts out a stem from a small tuber, and produces 

 a rosette of a few long leaves and one or more lateral roots, then lengthens into 

 a slender scape, and bears above a small-leaved spike of sporangia. The plant 

 is renewed by lateral adventitious shoots, consisting of a tuber and a leafless 

 rudiment of a bud; and in this respect resembles our native Orchideae. 



The S/em is distinguished in Isoetes, as has already been mentioned, by its 

 extraordinarily small growth in length, with which is connected, in this as in other 

 cases, an absence of branching ; no internodes ^ are formed, the leaves with broad 

 bases of insertion constituting a thick rosette, without leaving between them any 

 surface of the stem bare. The upper region of the stem, which is furnished with 

 leaves, has the form of a shallow funnel, depressed in the centre or apex (Fig. 305). 



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FIG. 305.— Longitudinal section of Isoetes lacustris at right angles to the fork of the stem ten months old (after Hofmeister)> 

 5 stem, b\. — *8 leaves, rl—rlO roots (X30) ; the ligula of the two developed leaves is shaded. 



The increase in thickness, the long continuance of which distinguishes the stem of 

 Isoetes from that of all other Cryptogams, is brought about by a layer of meristem 

 lying inside, surrounding the central vascular body, and continually producing new 

 layers of parenchyma on the outside. This takes place especially in two or three 

 directions, so that two or three corresponding masses of tissue are formed, slowly 

 dying oif on the outside, between which lie as many deep furrows meeting on the 

 ventral side of the stem. From these a large number of roots are produced in rows 

 in acropetal succession. 



In Selaginella, Lycopodium, Tmesipteris, and Psilotum, the stem remains 

 slender, but lengthens rapidly by a great number of dichotomies, and forms distinct 



The same occurs in Ophioglossacese and the short tuberous Cactacese. 



